Checking for a "stage" comment in lfJava is currently done in a very brittle manner, looking for a GITHUB_COMMENT containing the word "stage". This should be outsourced to a common function of some type, so that we can have Github and Gerrit functionality side by side, and potentially add other hosts (e.g. Gitlab) in the future.

Additionally, the comment keyword should be parameterized in such a way that the implementing pipeline can define what keyword they want. If one project uses "stage" and another uses "stage-release", the function should still work.