Implement server-to-server /token endpoint where vp_token is posted

[woutslakhorst]

Implements nuts-foundation/nuts-specification#265 (RFC021)

Things to do in order of the flow

• internal endpoint for requesting an access token (new version) at your own Nuts node #2484
• Resolve DID and find authorization server metadata #2487
• use presentation definition endpoint to get presentation definition from other party #2491
• handle presentation definition call and match with config, return definition, use presentation definition endpoint to get presentation definition from other party #2491
• match definition against wallet construct VP in the correct format (check metadata) #2504
• construct VP in the correct format (check metadata) #2504
• send VP to correct endpoint (from metadata) construct VP in the correct format (check metadata) #2504
• handle access token request with all correct VP validations #2495
• store and return access token
• support access token introspection (api and internals) #2541
• errors cases and error returns, check strictmode

[woutslakhorst]

note: the /token and /presentation_definition endpoints should be available in the oauth metadata.