Sts must be APIM ready

As customers want to run the secret-token-server behind an APIM we need to add a configurable map of headers to the request between the sts-client (sts-spring project) and the sts-server component. Therefore a configurable map of header-names and header-values should be implemented to add custom headers (e.g. client-ID : XXXXX-XXX-XXX, client-Secret XXXX-XXXX_Xxx).

To be able to deploy the sts-server the engineers must be able to access the swagger-ui.html. Currently the endpoints are not existing. The problem seems to be the outdated springfox dependeny. One must replace it with the newer springdoc dependency.

Acceptance criteria:
- Ability to add custom header keys/values
- Working and Accessable openapi-doc /swagger-ui.html