PKCE is enabled by default for new/updated auth methods.
* ref: https://oauth.net/2/pkce/
Client assertions are an optional, more secure replacement for client secrets
* ref: https://oauth.net/private-key-jwt/
a change to the existing flow, even without these new options,
is that the oidc.Req is retained on the Nomad server (leader)
in between auth-url and complete-auth calls.
and some fields in auth method config are now more strictly required.
