Merge branch 'develop' into 'master'

MEC028 v2.2.1 update - OAS & Proto3 representation updated

OAS & Proto3 representation have been updated to v2.2.1 specification.

Update followed [OpenAPI development guidelines](https://mecwiki.etsi.org/index.php?title=OpenAPI_development_guidelines)
- Generation using doc2oas *(a separate merge request has been created for the config file update)*
- Level-1 validation passing *(no warnings in Swagger-Editor, swagger-cli validate, speccy lint)*
- Level-2 validation passing *(no error in Swagger-UI, visual inspection)*
- Level-3 validation passing *(swagger code generation)*
- Proto3 generation passing
- Proto3 code generation passing
- Jenkins validation passing

See merge request !5