Implement OIDC Client for oAuth secured FHIR Stores

[UKFr-DIZ]

Given that in future the FHIR Server will enable oAuth Authentication mechanismn for access the FHIR Resources, the FHIR Gateway should be able to access FHIR Stores, which uses that mechanism.

There are Spring reference implementation from the MII Feasibility Process, which could be a good starting point.

Reference Implementation:
https://github.com/medizininformatik-initiative/mii-process-feasibility/pull/113/files