Make JWT key type independently configurable

[mbyczkowski]

Currently (including, the yet unreleased version 0.12) SPIRE has global configurable key type (ca_key_type config option) that allows to set RSA or EC key types for both JWTs and X509. This means that if one has to use RSA for JWTs, then X509 will switch to RSA as well, which might be undesirable.

This came up for me when looking into OIDC federation, during which I have learned that the vendor doesn't seem to support EC JWTs.

Perhaps as @evan2645 pointed in Slack, introducing a new config option (potentially called jwt_key_type) could be a great start here?

Long-term it would be good to be able to support multiple key types at the same time (at least for JWTs). I can see a scenario where a federation with vendor A could use RSA keys whereas federation with vendor B could use still use EC.