[XCM] Offchain tools and libraries

[acatangiu]

Description

As we've seen with pallet-xcm and xtokens, it's hard for onchain implementations to keep up with ecosystem cross-chain usage needs.

We should switch to leaner XCM pallets and richer TS/JS/Rust libraries and tools that build/expose many more flavors of XCM programs.

Through allowing on-chain execution of arbitrary XCM programs, we can build these useful XCM programs offchain, vend them as libraries and they can be used on any chain.

Developing XCM programs off-chain allows orders of magnitude improvements on: developers who could contribute, number of helpful programs provided, iteration speed, etc.

These programs can then be simply run on-chain using pallet_xcm::execute() or pallet_xcm::send().

Progress

• XCM Asset Transfer Program Builder #4736
• dry-running helpers/builders