add procedure for proto3 generation and validation

proto3-gen.md

+# Protobuf Schema Generation
+
+[OpenAPI Generator](https://openapi-generator.tech) is used to generate protobuf schema (`.proto3`) files from OpenAPI specifications of MEC012 RNI API.
+
+>**NOTE:** At the time of writing, the tool does not support OAS 3.1 version and we have to first convert the [RNI API](./RniAPI.yaml) to OAS 3.0 for generating protobuf schema.
+
+1. Convert OAS for [RNI API](./RniAPI.yaml) from 3.1 to 3.0​
+
+   - Change the value of `openapi` field from 3.1.0 to 3.0.0​
+
+   - Use this [VS code extension](https://marketplace.visualstudio.com/items?itemName=42Crunch.vscode-openapi) to see the errors in the downgraded YAML (v3.0)​
+
+   - Manually fix the errors​
+     - mostly related to `examples` <--> `example` interchange​
+     - or some 3.1 fields that are not supported in 3.0​ (comment them out)
+
+2. Generate proto files
+   - Install the `openapi-generator-cli.jar` using the installation procedure mentioned [here](https://openapi-generator.tech/docs/installation#jar).
+   - Generate the proto files using the following command 
+        ```sh
+        $ java -jar openapi-generator-cli.jar generate -i RniAPI.yaml -g protobuf-schema -o proto3/ --package-name mec012
+        ```
+
+3. Carefully inspect the generated `.proto` files for any inconsistencies. Some of the things to look out for:
+    - Proto3 generated files for enumerations, structures containing allOf, oneOf, anyOf etc. may need to be touched manually
+    - Check that all the nested models are being _imported_ correctly in their parent models
+    - Remove redundant proto files
+
+
+4. Validate protobuf schema by generating code from proto3 descriptions in different languages. See [this section](#code-generation-from-proto3) for more details.
+
+# Code Generation from proto3
+
+Below are some code generation examples for Python, Go and Ruby. For other languages, please refer to https://grpc.io/docs/quickstart/.
+
+
+### Python
+
+1. Install the grpcio-tools package
+    ```sh
+    $ pip install grpcio-tools
+    ```
+
+2. Create a directory for generated Python stubs
+    ```sh
+    $ mkdir python-stubs 
+    ```
+
+3. Run the following commands from the root of the directory containing this README that you are reading.
+
+   - Models:
+  
+        ```sh
+        $ python3 -m grpc_tools.protoc -I./proto3 --python_out=./python-stubs ./proto3/models/*
+        ```
+    
+        The above command will generate .py files for all the data models in the ./models directory
+
+   - Services:
+
+        ```sh
+        $ python3 -m grpc_tools.protoc -I./proto3 --python_out=./python-stubs --grpc_python_out=./python-stubs ./proto3/services/rni-service.proto
+        ```
+
+        The above command will generate two files for the RNI service:
+        - _rni_service_pb2.py_: containing the python data models used in the RNI service file
+        - _rni_service_pb2_grpc.py_: containing all the classes and functions needed for the supported HTTP methods in the RNI API
+
+### Go
+
+1. Install protocol buffer compiler
+    ```sh
+    $ apt install -y protobuf-compiler
+    ```
+2. Install Go plugins for `protoc`
+    ```sh
+    $ go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.26
+    ```
+    ```sh
+    $ go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@v1.1
+    ```
+3. Update `PATH` so `protoc` can find the plugins
+    ```sh
+    $ export PATH="$PATH:$(go env GOPATH)/bin"
+    ```
+4. Define a go package by appending `option go_package = "./mec012";` in all .proto files like this:
+
+    ```Go
+    ...
+
+    syntax = "proto3";
+
+    package mec012;
+
+    option go_package = "./mec012";
+
+    import public "models/<xyz>.proto";
+
+    ...
+    ```
+5. Generate Go code for models and services
+    ```sh 
+    $ mkdir go-stubs
+    $ protoc --go_out=./go-stubs ./proto3/models/* -I./proto3
+    $ protoc --go_out=./go-stubs ./proto3/services/* --go-grpc_out=go-stubs -I./proto3
+    ```
+    > The generated `<data_model>.pb.go` files will contain all the protocol buffer code to populate, serialize, and retrieve request and response message types defined in the `models` folder.
+    > And the `rni_service_grpc.pb.go` will contain the stubs for the methods defined in the `rni_service.proto` file.
+
+
+### Ruby
+
+1. Install gRPC Ruby Plugin and required tools
+   ```sh
+   $ gem install grpc
+   $ sudo apt install ruby-grpc-tools
+   ```
+
+2. Generate code
+    ```sh
+    $ mkdir ruby-stubs
+    ```
+    
+    Run the following command to create Ruby modules for all the data models defined in the proto files.
+
+    ```sh
+    $ grpc_tools_ruby_protoc -I./proto3 --ruby_out=ruby-stubs ./proto3/models/*
+    ```
+    Run the following command to generate `rni_service_pb.rb` and `rni_service_services_pb.rb` files, containing stub and service classes for the endpoints and methods defined in MEC012 RNI service.
+
+    ```sh
+    $ grpc_tools_ruby_protoc -I./proto3 --ruby_out=ruby-stubs --grpc_out=ruby-stubs ./proto3/services/*
+    ```
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