fix: use unique ID for linked accounts (#3441)

- move OAuth-related fields off of api_keys into a new user_links table
- restrict users to single form of login
- process updates to user email/usernames for OIDC
- added a login_type column to users

codersdk/users.go

@@ -27,6 +27,7 @@ type LoginType string
 const (
 	LoginTypePassword LoginType = "password"
 	LoginTypeGithub   LoginType = "github"
+	LoginTypeOIDC     LoginType = "oidc"
 )
 
 type UsersRequest struct {