RFC 0511 - challenge/domain should be not required (or optional)

The current spec says that in offer-credential `challenge` and `domain` are REQUIRED.

There might be two issues:
- It contradicts with the request-credential message where the spec says that `challenge` and `domain` might be obtained OOB. If OOB transmission is allowed, then we should probably not require these two fields in the request-credential message.
- The DIF Credential Manifest specification can include a `presentation_definition` which is defined [here](https://identity.foundation/presentation-exchange/#presentation-definition). Further, the Presentation Definition MAY include (or MAY be the same as--[still unclear to me](https://github.com/decentralized-identity/presentation-exchange/issues/112)) a Presentation Request which allows to include `challenge` / `domain` parameters. So, instead of passing `challenge` / `domain` in the offer-credential as individual parameters, we could use the Presentation Request to pass `challenge` / `domain` to the Holder for the Presentation Submission.

But I might be missing something @llorllale ?