added skeleton for Policy Management OpenAPI

src/SOL012/PolicyManagement/PolicyManagement.yaml

+openapi: 3.0.2
+info:
+  version: 1.0.0-impl:etsi.org:ETSI_NFV_OpenAPI:1
+  title: SOL012 - Policy Management Interface
+  description: >
+    SOL012 - Policy Management Interface
+
+    IMPORTANT: Please note that this file might be not aligned to the current version of the ETSI Group Specification
+    it refers to. In case of discrepancies the published ETSI Group Specification takes precedence.
+
+    Please report bugs to https://forge.etsi.org/bugzilla/buglist.cgi?component=Nfv-Openapis&list_id=61&product=NFV&resolution=
+  license:
+    name: ETSI Forge copyright notice
+    url: https://forge.etsi.org/etsi-forge-copyright-notice.txt
+externalDocs:
+  description: ETSI GS NFV-SOL 012 V3.3.1
+  url: https://docbox.etsi.org/ISG/NFV/Open/Drafts/SOL012ed331_Protocol_Spec_for_Policy_Mgmt_Intface/NFV-SOL012ed311v002.zip
+
+security:
+  - OauthSecurity:
+      - all
+servers:
+  - url: http://127.0.0.1/nfvpolicy/v1
+  - url: https://127.0.0.1/nfvpolicy/v1
+
+paths:
+  "/api_versions":
+    $ref: '../endpoints/SOL012_endpoints.yaml#/endpoints/api_versions'
+