Mime-type is required for credential request attributes

[lauravuo-techlab]

Mime-type is falsely required for credential request attributes.

Following error is encountered if credential preview attribute mime-type is omitted:

Hyperledger.Aries.AriesFrameworkException: attr_1 mime type of  not supported

Steps to reproduce the behavior:
Run interoperability tests with AATH so that dotnet-agent is playing the issuer role, e.g.

./manage build -a acapy -a dotnet
./manage run -d dotnet -b acapy  -t @T001-RFC0036

Expected behavior
Omitted mime-type is accepted and handled as default value as stated in RFC-0036. Issuing of the credential succeeds.