Sign credential from issuer component

[OSchlienz]

The issuer component will create the credenitals (bpn, membership and framework) and send them to the issuer wallet:
see creation process in issuer component:

• Create BPN Credential ssi-credential-issuer#3
• Create Membership Credential ssi-credential-issuer#4
• Create Framework Credential ssi-credential-issuer#5

To align:

• Usage of existing endpoint credentials/issuer in MIW and holder DID, after adapting to JWT and secp256k1
• Hypothesis: schema validation is done in issuer component
• provide signed credential back to portal (download option?)
• storage in holder wallet (direct as currently or in a separate step by ithe issuer)