XCM v1 version notification stub

[gavofyork]

This is a minimally functional SubscribeVersion/UnsubscribeVersion implementation for XCM v1, which allows XCM v2 networks to query v1 networks' version and be notified when they upgrade to v2.

This doesn't contain any notification-on-upgrade code since it's unneeded: when they upgrade to v2, then #3736 will be in place with its notification code. The only thing this PR does is ensure that the storage item for all notification targets is maintained ready for #3736 to use when it gets upgraded to.

Migration

XCM Config

Ensure that any xcm_executor::Config implementations includes a definition for the new type SubscriptionService. Generally this should be set to PalletXcm, the pallet_xcm concrete type. i.e.:

impl xcm_executor::Config for XcmConfig {
	/* snip */
	type SubscriptionService = XcmPallet;
}

Barrier

An additional barrier AlllowSubscriptionsFrom should be included in your Barriers tuple. You can probably set the origin filter to Everything, unless you want to filter which locations can ask to be notified of your chain's XCM version changes:

pub type Barrier = (
	TakeWeightCredit,
	/* snip */
	AllowSubscriptionsFrom<Everything>,
);

[gavofyork]

Not going to fix any more of the spelling.

[tomusdrw]

@TriplEight please remove bridges code from any checks, this is just a git subtree of the main respository, and it should not be spellchecked, cargo --fmt checked or anything else.