Allow listing callable names for a servide descriptor

[rnett]

Is your feature request related to a problem? Please describe.

It's not possible to get the callables registered with a service descriptor. This limits what services can be integrated with - they can only integrate with things that don't need any kind of discovery up-front. In particular, I was looking into whether it would be feasible to use Kotlinx RPC for annotation-defined MCP servers, and this is the only thing making it not possible.

Describe the solution you'd like

A method to get the callable names, or just the callables themselves, from a service.

Describe alternatives you've considered

There aren't really any.

Additional context

This would open up integrating with various things besides just networking clients. The RPC service would be a nicely-defined list of kotlin methods with input and output schemas.

[rnett]

@Mr3zee Assuming that all it would take is making the generated callableMap field public & adding it to the interface, I can do this in a PR if you like.