* Revert "Revert "experimentation: add endpoint that returns existing experiment if needed (#1188)" (#1204)"

An attempt to fix test flakes on main

Adds implementation of a experiment monitoring framework that supports specifying a number of TerminationCriteria that will
be evaluated periodically against active experiments, giving them a chance to terminate experiments based on arbitrary
conditions.

This framework relies on the termination action being idempotent: multiple workers might be monitoring the same experiment,
but only the first one to trigger will actually mutate the response, as the termination action is a noop if the experiment is no
longer running.

This PR does not introduce any of the configuration setup necessary to actually configure this for a Clutch deployment,
only the logic for polling active experiments and spawning per experiment goroutines.

This reverts commit aaf46d76.

* Add endpoint that creates or fetches an existing experiment depending on whether an experiment with specified experiment run already exists or not

Adds a test helper that blocks until Envoy is able to re-establish its xDS stream.
This should help de-flake individual test runs and avoid having to account for the
exponential backoff in other test assertions.

This reverts commit d6bebe49.