feat: reunify discover endpoints

[migmartri]

Currently, the referrer API service contains two endpoints, one public and one private

chainloop/app/controlplane/api/controlplane/v1/referrer.proto

Lines 26 to 33 in ab86233

	service ReferrerService {
	// DiscoverPrivate returns the referrer item for a given digest in the organizations of the logged-in user
	rpc DiscoverPrivate(ReferrerServiceDiscoverPrivateRequest) returns (ReferrerServiceDiscoverPrivateResponse);
	// DiscoverPublicShared returns the referrer item for a given digest in the public shared index
	rpc DiscoverPublicShared(DiscoverPublicSharedRequest) returns (DiscoverPublicSharedResponse) {
	option (google.api.http) = {get: "/discover/{digest}"};
	}
	}

The private one returns information from any of your organizations while the public one only information stored in the public index.

There is a common use-case where we want to call both the private and the public one and merge the results, this requires two API calls.

We could look into extending the private endpoint to support a flag that says smth like include_public that internally will make sure it also fetches the info from the public index.

We could look into having a single endpoint but that will require more work since one of them does not have any authorization, and will require many core changes