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

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* Spelling

* Unit tests for XCM executor

* Formatting

* Spellin

* Unit test for XCM pallet subscriber side

* Formatting

* More tests

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* Fixes

* Subscription-side tests

* Formatting

* Unit tests for XCM pallet

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

Co-authored-by: Shawn Tabrizi <[email protected]>

* Remove commented code

* Grumbles

* Multi-stage XCM version migration

* Formatting

* v1 subscriptions backport

* Warning

* Spelling

* Fix grumbles

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* Avoid running through old notifications

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Co-authored-by: Shawn Tabrizi <[email protected]>