introduce key identifier

[strantalis]

To help support key rotation within KAS a key identifier must be introduced. This will involve work in the kas service itself. Along with any sdk.

[dmihalcik-virtru]

This is a feature where KAS can store multiple key pairs, and is essential for supporting key rotation, necessary for a production platform, and would enable other advanced key management capabilities.

We need to

1. Allow KAS to load multiple keys of different types, and associate them with KIDs
2. Update all clients to embed the kid in their documents (both nanotdf and ztdf)
3. Perform any changes to the protocol required to enable these features.

Due to the way TDF is currently structured, once a file is created it is pinned to the given public key itself. There are extensions that allow some degree of forward secrecy around this -

Questions:

1. Should we support JWKS from the public key endpoint? If so

[dmihalcik-virtru]

I'll look into extending the configuration format to allow supplying a matrix of key types and identifiers.

[dmihalcik-virtru]

We still need to add kid to the nanotdf file format, but I have code testing it in #969