[yang]SONiC Yang model support for Telemetry_client. (#12483) (#13314)

#### Why I did it
Create SONIC Yang model for Telemetry_client
#### How I did it
Defined Yang models  based on Guideline doc:
https://github.com/Azure/SONiC/blob/master/doc/mgmt/SONiC_YANG_Model_Guidelines.md
and
https://github.com/Azure/sonic-utilities/blob/master/doc/Command-Reference.md
#### How to verify it
Added test cases to verify it.