add -service=XX cli flag to only perform gen on specific service and related types

[pkieltyka]

it would be useful for client generation, and server generation too in some cases, to pass -service=API where "API" is the service API name in a webrpc schema and it would only include the code-gen of types and interfaces which are used by the "API" service rpc methods.

this is useful so you can have a schema with both "API" and "Admin" service, and generate a public client for just the "API" and a separate internal client for the "-service=API,Admin" with both, or just omit -service for standard generation of all, or admin only as -service=Admin