XCM: Automatic Version Negotiation (#3736)

* XCM: Automatic Version Negotiation

* Introduce the version instructions and subscription trait

* Notification and subscription data migration

* Version change subscriptions

* Fixes

* Formatting

* Spelling

* Fixes

* Fixes

* Automatic unsubscription

* Formatting

* Expose remote origin in VM and ensure it is unchanged from actual origin in subscription instructions.

* Barrier

* Unsubscription extrinsic

* Remove top_level param

* Formatting

* Fixes

* Automatic subscription

* Formatting

* Spelling

* Unit tests for XCM executor

* Formatting

* Spellin

* Unit test for XCM pallet subscriber side

* Formatting

* More tests

* Formatting

* Fixes

* Subscription-side tests

* Formatting

* Unit tests for XCM pallet

* Formatting

* Update roadmap/implementers-guide/src/types/overseer-protocol.md

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Remove commented code

* Grumbles

* Multi-stage XCM version migration

* Formatting

* v1 subscriptions backport

* Warning

* Spelling

* Fix grumbles

* Formatting

* Avoid running through old notifications

* Formatting

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

polkadot/xcm/src/v2/traits.rs

@@ -99,6 +99,8 @@ pub enum Error {
 	Trap(u64),
 	/// The given claim could not be recognized/found.
 	UnknownClaim,
+	/// The location given was invalid for some reason specific to the operation at hand.
+	InvalidLocation,
 }
 
 impl From<()> for Error {