Ensure that we may remove extra folders from bridges subtree in Cumulus

[svyatonik]

We may want to remove unneeded folders from the bridges subtree in Cumulus. We only need pallets, their primitives and runtime-common. But e.g. runtime-common has references to our testnets, so we can't simply remove testnet runtimes.

We need fix it (remove backreferences) and prepare some script that we may use when updating our subtree in Cumulus that will only leave required folders.

[bkontur]

I started with this: https://github.com/paritytech/cumulus/blob/bridge-hub-rococo-wococo/scripts/bridges_update_subtree.sh

[svyatonik]

Closed #1832: ./bridges/scripts/verify-pallets-build.sh --no-revert && then git commit -a -m "Remove extra folders".

[bkontur]

verify-pallets-build.sh is included in cumulus update subtree notes/process:
https://github.com/paritytech/cumulus/blob/bridge-hub-rococo-wococo/scripts/bridges_update_subtree.sh#L48
https://github.com/paritytech/cumulus/blob/bridge-hub-rococo-wococo/parachains/runtimes/bridge-hubs/README.md#how-to-update-bridges-subtree

I was able to create subtree update with:

./scripts/bridges_update_subtree.sh fetch
./scripts/bridges_update_subtree.sh patch (runs `verify-pallets-build.sh`)
./scripts/bridges_update_subtree.sh merge

+ some manual conflict resolution

but it is not that straight because git subtree pull creates conflicts with previously deleted files, so ./scripts/bridges_update_subtree.sh merge just deletes them and simplify conflict resolutions,
but at the end of merge process ./scripts/bridges_update_subtree.sh patch have to work and it does, so cool :)

anyway, merge conflicts will just depend on subtree diff

[bkontur]

@svyatonik
wdyt about runnning partial-repo-build-test on all PRs?

I am just thinking, we should be able to release subtree whenever, so maybe this check should run on all PRs, ideally, it should run, if PR touches relevant files, but that is very very nice to have :)

or is there any way how to trigger partial-repo-build-test step manually in CI?