Improve the PoC agent invocation method

[vigoo]

As part of the bridge epic we've created a new agent invocation endpoint, that relies on a new gRPC endpoint in registry service to find the current deployed agent metadata of a given app/env by names. Review the implementation of these new PoC endpoints and make them production ready (introduce caches / new queries if needed)

[mschuwalow]

The registry service side is also missing some permission checks here

The actual users token needs to be sent to the registry service. While it can use AuthCtx::System internally to simplify the resolution logic, it needs to check that the final resolved agent type (via environment ownership + shares) is accessible to the requesting user.

[noise64]

while this is done it should be tested:

• with trigger (currently results in cancellation)
• with schedule (currently results in "not implementd")