feat(bigtable): add 'client_options' support to Client

[emar-kar]

Towards #8475

List of changes:

1. client:
   • as BigTable Client creates by GAPIC client_options were integrated into the _create_gapic_client method call.
2. setup:
   • google-cloud-core version updated on the basis of the comment;
   • the file was reformatted by black.
3. test_client:
   • since api_endpoint is an irrevocable argument, tests check the correctness of the API call;
   • by default client_optionsshould be None.

[tseaver]

This PR has several issues:

• Bigtable has two endpoints, one for table data and one for table / instance admin, and therefore the client constructor needs both a client_options and an admin_client_options argument.
• The correct client_options / admin_client_options arguments must be passed down to the GAPIC class, which means that _create_gapic_client must take that value as an argument, rather than looking it up on the instance.

PR #9516 corrects those two issues.

Also, please avoid intermingling janitorial changes (blackening setup.py) with substantive ones.