Add precommit checks to look for secrets and other stuff

[boredabdel]

While reviewing #543 @jaketf had a great suggestion about using precommit-checks https://pre-commit.com/ to look for secrets (like private SA keys) and maybe even enforce license and stuff like that.

I'm not sure how the repo is configure but will have a look at this.

[jaketf]

@boredabdel I have a decent amount of experience w/ pre-commit.
This example (PSO internal CSR link) might help get you started. I'd be happy to discuss / help if you need. The biggest challenge is the fact that we have so many assets in the exclusion list.

[mik-laj]

We have separate repo for pre-commit hooks. https://github.com/google/pre-commit-tool-hooks is it interesting for you?

[jaketf]

@mik-laj very interesting! thank you for pointing this out!