XCM extensions

[xlc]

We should have optional XCM extensions for features that useful for some chain but not every chain will implement it

Required for #11, #12, #14

Also may need feature discovery to detect available features on destination chain. But it could just be a registry somewhere offchain.

[xlc]

What should be the strategy of including new XCM extensions?

I see few options:

---

Add new message type for each XCM extension

pub enum Xcm {
	// existing types
	Staking(StakingMessage),
	Governance(GovernanceMessage),
	Storage(StorageMessage),
}

---

Add just one message type for all extensions

pub enum Xcm {
	// existing types
	Extension(XcmExtensionMessage),
}

enum XcmExtensionMessage {
	Staking(StakingMessage),
	Governance(GovernanceMessage),
	Storage(StorageMessage),
}

---

Or something very generic

pub enum Xcm {
	// existing types
	Extension { extension_id: Vec<u8>, extension_payload: Vec<u8> },
}

[xlc]

Had a chat with Gav, will just use the first approach.