Move transactions protocol to its own crate

[altonen]

Move transactions protocol to its own crate.

polkadot companion: https://github.com/paritytech/polkadot/pull/5973 cumulus companion: https://github.com/paritytech/cumulus/pull/1661

[altonen]

This PR supersedes the previous #12198 as the history in that PR was messed up and fixing it would require force-pushing

[acatangiu]

bot merge

[paritytech-processbot[bot]]

Error: Github API says https://github.com/paritytech/polkadot/pull/5973 is not mergeable

[acatangiu]

bot rebase

[paritytech-processbot[bot]]

Branch is already up-to-date

[acatangiu]

bot merge

[acatangiu]

after doing "bot merge" PR updated showing recently added "A1-onice" label; alas bot has already merged it...

@altonen why is this not ready for merge?

[altonen]

I wanted to double-check one thing but I guess it's OK. I don't understand why the Polkadot PR didn't merge and if somebody knows how to fix it, I'd appreciate help

[acatangiu]

I think extra commit went into it after the CI had originally run the companion, just a guess.

We'll see if bot wants to merge once CI goes green, the lockfile was updated correctly.