Support initial bidirectional stream messages.

[samuel-williams-shopify]

Summary

• Adds an initial: option to Async::GRPC::Client#invoke for bidirectional streaming calls.
• Writes initial bidirectional messages before waiting for response headers.
• Passes initial: through dynamic stubs.
• Covers the server-waits-for-first-client-message case.

Testing

• fish -l -c 'bundle exec sus test/async/grpc/client.rb'
• fish -l -c 'bundle exec sus'