Support transport-neutral TLS credentials

[samuel-williams-shopify]

Summary

• add readable channel credentials matching grpc-ruby's positional constructor arguments
• translate custom trust roots and client identities into IO::Endpoint::TLS::Configuration
• pass the TLS configuration through async-http v0.100.0
• retain native grpc-ruby credentials for ordinary TLS while documenting that their certificate material is opaque
• add custom-root and mutual TLS integration coverage

Verification

• bundle exec bake test
• COVERAGE=PartialSummary bundle exec bake test (100%, 163/163 lines)
• bundle exec rubocop
• bundle exec bake gem:build