POC: Add built in steps to workflows

[jdelfino]

I imagine this interface change would require some more discussion and documentation, so I'm not looking to submit this, but wanted to send for discussion.

This change would support custom setup for kythe/codesearch indexing. It would unfortunately remove "automatic" injection of these steps, and require anyone using codesearch to inject boilerplate steps into their workflow config.

The main alternative that I can think of to this approach would be a separate "codesearch" top-level section in the workflow config that allowed specification of setup steps required for a kythe build, or maybe even just packages to install or a docker container to use. The adoption curve for that would be gentler - if no custom setup steps were required, kythe & CS indexing would continue to "just work" when codesearch was enabled.

This is based on GH Actions uses: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstepsuses

[bduffany]

I like this - in the future I hope uses can support custom reusable actions, so it might be good to format this as a git remote file reference from the start, like buildbuddy-io/actions/kythe-index, buildbuddy-io/actions/codesearch-index. For now we can hard-code the actual configs, but in the future, we could transparently swap out the implementation so that it fetches from the repo github.com/buildbuddy-io/actions and looks for ./codesearch-index/manifest.yaml or something like that (can be done in a single API request, similar to how we fetch buildbuddy.yaml). Ideally, users would also specify a commit_sha but we could maybe default to HEAD if one isn't specified.