Support gRPC default authority

[chenrui333]

Summary

• add a default_authority connection option backed by grpc.default_authority
• apply the authority to both workflow and operator service channels
• document absolute resolver targets with a separate TLS/HTTP2 authority

This allows clients to use a DNS target with a trailing dot while preserving
the certificate and HTTP/2 authority of the canonical undotted hostname.

Tests

• bundle exec rspec spec/unit/lib/temporal/grpc_spec.rb -e default_authority
• full suite adds no failures relative to current master under Ruby 3.2.2