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/double_encoded.rs

@@ -25,6 +25,7 @@ use parity_scale_codec::{Decode, DecodeLimit, Encode};
 #[codec(decode_bound())]
 #[scale_info(bounds(), skip_type_params(T))]
 #[scale_info(replace_segment("staging_xcm", "xcm"))]
+#[cfg_attr(feature = "json-schema", derive(schemars::JsonSchema))]
 pub struct DoubleEncoded<T> {
 	encoded: Vec<u8>,
 	#[codec(skip)]