Allow self attested attributes when answering a presentation request

[etschelp]

See also: https://github.com/hyperledger-labs/business-partner-agent/issues/650

In case the presentation request has no restrictions on the issuer, or issuer is holder, the request can also be answered with self attested attributes. This could be a very nice feature if we make it document based, as this avoids connecting the agent to itself to get the wallet credential.

Dif:

Either now restrictions or subject_is_issuer set.