GONRG-9652: add group members count in JDBC

Type of change

• Bug Fix
• Feature

Does this introduce a change in the core logic?

• [NO]

Does this introduce a change in the cloud provider implementation, if so which cloud?

• AWS
• Azure
• Google Cloud
• IBM

Does this introduce a breaking change?

• [NO]

What is the current behavior?

Calling the endpoint /groups/{group_email}/membersCount produced NotImplementedException and resulted in 500 error for client.

What is the new/expected behavior?

With this MR endpoint returns count of users and subgroups for requested groupId, role and partitionId.

Have you added/updated Unit Tests and Integration Tests?

• [YES]