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.