Support using JWT/nkey credentials from a creds file

[autodidaddict]

Current authorization settings support supplying a user's JWT and nkey seed individually but do not support supplying a path to a credentials file. It shouldn't take much to add this.

Additionally, it looks like JWT and nkey seed might not be in the settings typespec, so we should fix that while we're in there.

[mmmries]

We should also tackle #160 whenever we make these changes by only grabbing the credentials from a filepath or an environment variable at the time of the handshake, that way we won't be keeping sensitive credentials in the process state and we won't have to worry about accidentally logging those credentials