identity: Don't let subs be re-registered (#6667)

* Fixes and tests

* Don't set subs be re-registered.

Also allow subs to de-register themselves and collect the deposit.

Also allow individual registering and removal of subs.

* Make it build

* Update frame/identity/src/lib.rs

Co-authored-by: joe petrowski <[email protected]>

* Tests

* Add benchmarks

* Add some reasonable weights

* Docs

Co-authored-by: joe petrowski <[email protected]>

Author: gavofyork

src/lib.rs

@@ -172,7 +172,7 @@ mod tests {
 	}
 
 	impl_outer_origin! {
-		pub enum Origin for Test  where system = frame_system {}
+		pub enum Origin for Test where system = frame_system {}
 	}
 
 	pub struct TestSessionHandler;