sync discovery service spec and implementation

[gerardsn]

Add to implementation from spec (§3.4):

• retractions must have type RetractedVerifiablePresentation
• retraction and retracted VP must have the same expiration time
• add support for PEX relational constraint feature is_holder and same_subject

change in spec (§4):

• update requirement all credentials MUST have the same credentialSubject.id. If this is required, the PD should use PEX relational constraint feature same_subject
• update requirement the key used to sign the presentation MUST be owned by the credential subject (see 4.1): the JWT kid header MUST reference an assertionMethod key from the subject's DID document.. Part of the requirement sounds like relational constraint feature is_holder .
• add clarification on how all credentials in a VP are tied together

[woutslakhorst]

• Add DiscoveryRegistrationCredential

[woutslakhorst]

is_holder and same_subject will not be implemented.

[woutslakhorst]

see nuts-foundation/nuts-specification#267