Create a custom GitHub action for re-usable rain CI across repos

[0xgleb]

We have a lot of duplication in CI across repos and we don't want to maintain a bunch of nearly identical pipelines. At the same time, CI does have different needs depending on the repo, e.g. different git submodules need to be prepared for different repos, and depending on the pipeline, e.g. rust pipelines rely on solidity artifacts and typescript pipelines often rely on rust wasm artifacts.

To address this issue we can create a custom GitHub action (or potentially multiple) that pull out the commonalities while providing enough configuration options to account for the different CI needs.

We have had situations where CI changes are included with other changes, which causes CI setups to diverge across repos/pipelines. If the code responsible for CI lives in a specific place and controls all the repos then that also would prevent CI changes from being slipped in with other changes. It would necessitate a separate PR and an independent review for CI changes, which can then be rejected separately from all the other changes if there isn't a good enough reason to update CI