FEP-ef61 updates:

- Replaced did:apkey with did:ap:key. With did:ap as a base, other similar DID methods can be constructed. For example did:ap:web can be used to delegate key management to a web service.
- Accordingly, resolver endpoint was changed from .well-known/apkey to .well-known/apresolver
- Added image attachment to object example. Objects have a signature that proves their authenticity, but attachments can't be verified in the same way. digestMultibase property solves this problem.

I also updated all related FEPs:

- FEP-8b32
- FEP-c390
- FEP-ae97