Make connection to an external trusted third party for token generation and validation

[JackJackie]

The validation of tokens and provision of requester IDs is now enabled via a token-to-requester json file in the SPARQL endpoint itself. This should be done by a Trusted Third Party.

• Such a party should be identified
• An API for token generation should become available by the TTP
• An API for token validation should be made available by the TTP
• The endpoint should check an incoming token for validity and ask for the ID of the corresponding requester for further usage.