APIs for managing custom search attributes

[jeffschoner]

Summary

Adds add, list, and remove APIs for custom search attributes. Since these operations live on the OperatorService in the GRPC contract, a client for this has also been introduced into grpc.rb.

Testing

New unit specs at client and GRPC connection level:

bundle exec rspec spec/unit/lib/temporal/client_spec.rb:790:804:819
bundle exec rspec spec/unit/lib/temporal/grpc_spec.rb:649:733:789

New example specs:

bundle exec rspec spec/integration/search_attributes_spec.rb