Automatic merge from submit-queue (batch tested with PRs 43871, 44053)

Proxy healthchecks overhaul

The first commit is #44051 

These three commits are tightly coupled, but should be reviewed one-by-one.  The first adds tests for healthchecks, and found a bug.  The second basically rewrites the healthcheck pkg to be much simpler and less flexible (since we weren't using the flexibility).  The third tweaks how healthchecks are handled in endpoints-path to be more like they are in services-path.

@MrHohn because I know you were in here for source-IP GA work.

@wojtek-t