feat(xcm): support json schema (for CosmWasm VM support) (#1454)

# Description

- What does this PR do? Allows to generate JSON schema for subset of XCM
in std builds
- Why are these changes needed? To support XCM messages in CosmWasm
contracts which require Schemars to generate contract clients
- How were these changes implemented and what do they affect? We will
use schema feature flag to build XCM pallet with JSON schema enabled

# Checklist

- [x] My PR includes a detailed description as outlined in the
"Description" section above
- [x] My PR follows the [labeling requirements](CONTRIBUTING.md#Process)
of this project (at minimum one label for `T`
  required)
- [x] I have made corresponding changes to the documentation (if
applicable)
- [x] I have added tests that prove my fix is effective or that my
feature works (if applicable)
- [x] If this PR alters any external APIs or interfaces used by
Polkadot, the corresponding Polkadot PR is ready as well
  as the corresponding Cumulus PR (optional)

polkadot/xcm/src/v3/multilocation.rs

@@ -66,6 +66,7 @@ use scale_info::TypeInfo;
 	serde::Serialize,
 	serde::Deserialize,
 )]
+#[cfg_attr(feature = "json-schema", derive(schemars::JsonSchema))]
 pub struct MultiLocation {
 	/// The number of parent junctions at the beginning of this `MultiLocation`.
 	pub parents: u8,