Add "GAIA-X trust framework" context(s)

Ass described in issue 200, the latest VCs out of the GAIA-X wizard can not be inserted into the catalog, since the used context is unknown and will not be loaded for security reasons see here. Therefore it may be necessary to add additional context(s) to the catalog to make the verification and validation of VerifiablePresentations possible.

Additional contexts can be configured now by the application.yml configuration file and will be added to the VPs during the verification process. Therefore a new config item federated-catalogue.verification.additional-contexts was added to the application configuration. A CachableHttpLoader is used to reduce connectivity. Still, we need to think about it, if we should add the context files into the catalog binaries and load it from the application itself rather than streaming it from the trustframework. Have a local copy may increase speed and make it more failsafe. Right now I expect further changes within the context files, so a configurable solution, loading the files on demand is the preferred way for the next couple of months/years.