Use options for binding services

[emcfarlane]

Idea for keeping service bindings as Options. ServiceOption no longer implements TranscoderOption. To add default service options call: WithDefaultServiceOption(svcOpts...). This lets us hide the *Service implementation and move NewService and NewServiceWithSchema to WithService and WithServiceFromSchema.

vanguardgrpc no longer returns a http.Handler but instead creates a new option for the transcoder:

handler, err := vanguard.NewTranscoder(
    vanguardgrpc.WithGRPCServer(server),
)

[emcfarlane]

Feel free to close @akshayjshah

[emcfarlane]

Closing, talked over with Akshay. Having the services not truly be optional is the main downside. This approach mixes the boundary too far.