[XCMv5] Better fee mechanism (#5420)

Implements [RFC#105](https://github.com/polkadot-fellows/RFCs/pull/105)
which, at the time of this PR, has not been approved yet. Some aspects
might be changed as a result of discussion.

## TODO
- [x] Add new instruction and use it in conversion functions
- [x] Implement in xcm-executor
- [x] Setup for xcm-executor unit tests
- [x] Actual xcm-executor unit tests
  - [x] Happy path
  - [x] Unhappy path
- [x] Emulated tests
  - [x] Asset hub westend
  - [x] Asset hub rococo
- [x] Benchmarks
  - [x] Dummy values
  - [x] Actual benchmarks
- [x] PRDoc

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Co-authored-by: command-bot <>
Co-authored-by: Adrian Catangiu <[email protected]>