Automatically get latest gRPC source

[henderkes]

We need to use type 'git' because we need to clone submodules. But we only have 'rev' for that, which does not accept version constraints like 1.\d+.\d+.

We should look to find a way to update grpc source without always updating the '1.75.x' rev.

[crazywhalecc]

It sounds like a necessary operation, but the only reasonable way I can think of is to write a built-in source update command and then let actions automatically refresh source.json.

In this case, we may need to implement a metadata interface for source. I'm not sure how to implement it for now, but I've recently drafted a change list for 3.0, I can add to it.

And if we use Git commands to query, AFAIK it can only pull a main branch first and then query the branch list and switch, and we can't do this with GitHub's API because there is not only GitHub's Git repository.

[henderkes]

I think using git commands to query branches, match against a regex and then checkout is a viable strategy.