Allow composition of multiple generated CLI definitions into one entrypoint

[SRHerzog]

From Slack:
it would be cool to add one more positional CLI argument to choose which service to call and have a single file as the entrypoint, instead of a file for each individual manager

Right now, the CLI generation code works from a single OpenAPI specification that makes a single instance of HttpClient. It would be nice to be able to pass in multiple OpenAPI specs or multiple HttpClients and make a single CLI to access all of them.