Allow API clients to choose which key type to sign JWTs with

[MicahParks]

Currently, JWTs are only signed with whatever the default key is in the database. Using the default configuration, this is the EdDSA key.

Add an option to all client requests to specify a signing key type for the JWT on a per-request basis. This supports the use case when a client application does not support the default key type.

[MicahParks]

Perhaps this should be the alg header value instead, as they kty is less specific.