🎨 Standardise issuer and verify send/accept endpoint convention

[ff137]

Extending from #226:

issuer uses POST /generic/issuer/credentials for send offer and /generic/issuer/credentials/{credential_id}/request for send request
verifier uses POST /generic/verifier/send-request and /generic/verifier/accept-request with the proof id in the body.

It's a sound recommendation that these endpoints should follow a unified convention.

I recommend that the issuer endpoints should be more like the verifier routes:

• POST /generic/issuer/credentials -> /generic/issuer/credentials/send-credential or /generic/issuer/send-credential
• POST /generic/issuer/credentials/{credential_id}/request -> /generic/issuer/credentials/accept-credential