Feature/configurable userid claim minimal (#499)

* Add -user-id-claim to support other claims than email

Fix #431 - This is a minimal change to allow the user to configure which claim is
the source of the "user ID".

- Add the option `user-id-claim` (defaults to email)
- OIDC extracts this claim into session.Email (to be renamed later)
- providers: add `CreateSessionStateFromBearerToken` with a default impl taken from
  `GetJwtSession` and overridden by oidc to respect `user-id-claim`

Once #466 is merged, I can continue to rename SessionState.Email to .UserID
and add HTTP headers with a corresponding name.

* Apply suggestions from code review

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

* Review feedback: Don't extract claims manually

Instead, parse them twice - it might be sligtly slower but less bug-prone as the code evolves.

* Fix indentation

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

main.go

@@ -147,6 +147,8 @@ func main() {
 	flagSet.String("pubjwk-url", "", "JWK pubkey access endpoint: required by login.gov")
 	flagSet.Bool("gcp-healthchecks", false, "Enable GCP/GKE healthcheck endpoints")
 
+	flagSet.String("user-id-claim", "email", "which claim contains the user ID")
+
 	flagSet.Parse(os.Args[1:])
 
 	if *showVersion {