What this PR changes/adds

this PR changes the following things:

• CredentialObject now has an id property
• CredentialOfferMessage can contain either the fully-fledged CredentialObject, or just an {"id": "..."} object.
• CredentiaRequestMessage contains only a list of {"id": "..."} objects referencing the CredentialObject

Why it does that

Compliance with latest TCK changes

Further notes

• the Issuer Admin API was also changed such that the credential offer endpoint takes an array of IDs instead of a CredentialDescriptor.

Who will sponsor this feature?

Please @-mention the committer that will sponsor your feature.

Linked Issue(s)

Closes # <-- insert Issue number if one exists

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