Deprecate usage of the 'proof' parameter in the credential request

[tplooker]

With the introduction of #293 and the proofs parameter there is an opportunity to simplify the credential request syntax to deprecate the usage of the proof parameter and instead just use the proofs parameter.

[tplooker]

The same conversation applies to the credentials vs credential parameter in the credential response syntax.

[babisRoutis]

I think that now it is good opportunity to make this simplification, since implementers have to remove support to batch endpoint and adjust their support for the credential endpoint, in any case.

[jogu]

Removing credential would require an update to the deferred issuance endpoint that currently references that definition (but an update to deferred issuance endpoint to mention credentials is required anyway, so this is easy to solve).

[c2bo]

I'd like to bump this discussion before we go to 1.0 - now is a good time to simplify and there are a few breaking changes happening anyway. The issue with deferred issuance was fixed in #391.