Add CI for submodules updates

[nkaskov]

We constantly need to test our updates. The problem is that updates in submodules may depend on each other - and we need to test it before merging the updates.

Currently, we need to do it manually, which gives us a lot of pain.

A perfect user story looks like that:

1. Developer starts developing from an issue in zkllvm repository
2. (Assuming that this issue requires changes in both assigner and circifier) - developer makes changes in feature-branches and creates PRs in all repos (except from zkllvm repository - because zkllvm PR requires merging PRs in submodules first)
3. Dependent PRs are being detected by a bot. It runs dependent testing on corresponding branches.
4. If all good - Bot approves all PRs
5. Another developer approves these PRs.
6. PRs in submodules rebase to master
7. Bot creates PR in zkllvm (I am not sure if we need this step).

Maybe this can be done by giving branches the same names. It is not very convenient, and it also depends on human factor.

Another option - keywords or labels in PRs. This would require a minimum job done by the developer to trigger correct bot behavior.

[nil-foundation-cicd-bot]

PRs, synced with the current issue:

• NilFoundation/zkLLVM#394 (SHA: 2fa12fe)

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