Draft: Handling shared schemas in azure

Key Highlights:

1. Introduced azure only API for creating shared schemas. The API contract is same as regular schema create API. The URL is <HOST>/api/schema-service/v1/sharedSchema. Previously it was <HOST>/api/schema-service/v1/schema
2. Moving away from shared tenant notion. Now shared schemas for azure won't be put into any common tenant. Instead those would be put into a pseudo partition named system. This system is internal partition and not accessible via APIs. 3.For the code in schema-core, the sharedTenant would still be present, hence making it a non breaking change for other CSPs, but for azure this shared tenant is same as system tenant only.

Feel free to comment on various classes, explaining your point of view.