System Tenant Changes for creating system schemas

Followed with partition!586 (merged) and partition#69 (closed)

We are adding change in schema service for Azure to validate a user against a hidden partition(not actual partition) 'system', the change is targeted only for Azure where customers wanted to be able to create system schema.

There is no api level changes, for normal users provisioned to be able to create system level schemas, i.e. a member of service.system-schema-service.editors in a dummy partition 'system'.

This is until a better end to end approach is finalized with /system endpoints in all the required services. When that happens, most of this code will become obsolete and may be removed in future.

The changes will not impact any other CSP, the application behavior is same for them as before. The changes do contain an interface and a default implementation including unit tests for the complete code to work.

Integration tests are not added as it needs users token validation and we cannot simulate that as of now in glab, we only have SA account which can anyway create system schemas.

cc: @nthakur, @lucynliu, @ashley_beitz, @ladatz, @jordanchiu