Link Gaia-X Credentials with each other

[anjastrunk]

Gaia-X Credentials may refer each other. E.g. A Service Offering refers Gaia-X Credential of CPS.

Currently, gx-credential-generator puts all Gaia-X Credentials in one big file. In case of a Service Offering, the file contains Gaia-X credential of both the service itself and its CPS. A more flexible approach would be to refer credentials via URIs.

Task: Implement reference of credentials via URI.

related to #73