Expose service identity as a did:web document

[achamayou]

See https://github.com/microsoft/scitt-ccf-ledger/issues/53 and https://github.com/microsoft/scitt-ccf-ledger/issues/88 for history.

This information is exposed now in the JSON & C++ receipts as "service_endorsements": https://github.com/microsoft/CCF/blob/b78c1d00133d3c2979d8f11ec10c34345e6bb3b4/include/ccf/receipt.h#L30, but not in a did/jwk-friendly format.

This is partially implemented in the SCITT ledger now, but would be useful to bring back to CCF generally, for any user who wants a service identity history in a different format. To be defined and implemented is the way key validity ranges are indicated, perhaps by defining additional possible values for use, such as "sig:from:2.54:to:5.7869", or by using a new field in the key.

[achamayou]

SCITT has now moved on from did:web, and instead specifies something much closer to openid-configuration, and SCITT/application-specific that does make sense in a generic CCF context:

2.1.1.  Transparency Configuration

   Authentication SHOULD NOT be implemented for this endpoint.  This
   endpoint is used to discovery the capabilites of a transparency
   service implementing this specification.

   Request:

   GET /.well-known/transparency-configuration HTTP/1.1
   Host: transparency.example
   Accept: application/json

   Response:

Birkholz, et al.        Expires 5 September 2024                [Page 5]
Internet-Draft                   SCRAPI                       March 2024

   HTTP/1.1 200 Ok
   Content-Type: application/json

   {
     "issuer": "https://transparency.example",
     "registration_endpoint": "https://transparency.example/entries",
     "nonce_endpoint": "https://transparency.example/nonce",

     "registration_policy": \
   "https://transparency.example\
   /statements/urn:ietf:params:scitt:statement\
   :sha-256:base64url:5i6UeRzg1...qnGmr1o",

     "supported_signature_algorithms": ["ES256"],
     "jwks": {
       "keys": [
         {
           "kid": "urn:ietf:params:oauth:\
   jwk-thumbprint:sha-256:DgyowWs04gfVRim5i1WlQ-HFFFKI6Ltqulj1rXPagRo",
           "alg": "ES256",
           "use": "sig",
           "kty": "EC",
           "crv": "P-256",
           "x": "p-kZ4uOASt9IjQRTrWikGnlbGb-z3LU1ltwRjZaOS9w",
           "y": "ymXE1yltJPXgjQSRe9NweN3TLlSUALYZTzy83NVfdg0"
         },
         {
           "kid": "urn:ietf:params:oauth:\
   jwk-thumbprint:sha-256:4Fzx5HO1W0ob9CZNc3RJx28Ixpgy9JAFM8jyXKW0ClE",
           "alg": "HPKE-Base-P256-SHA256-AES128GCM",
           "use": "enc",
           "kty": "EC",
           "crv": "P-256",
           "x": "Vreuil95vzR6ixutgBBf2ota-rj97MvKfuJWB4qqp5w",
           "y": "NkUTeaoNlLRRsVRxHGDA-RsA0ex2tSpcd3G-4SmKXbs"
         }
       ]
     }
   }

[achamayou]

FYI @ivarprudnikov @andpiccione @vimauro