* return experiment status
* Remove 'Stopped' experiment status
* Rename `id` to `Id`
* support null `termination_reasons`

Wires up the termination framework as a configurable service that allows configuring an arbitrary set of termination criteria,
including custom ones that can be registered by adding them to the terminator.CriteriaFactories map. This allows for
custom pluggable termination criteria that may rely on internal systems (e.g. check for pages using an internal library)
that can hook into the monitoring system provided by the termination framework.

Includes a single termination criteria as an example, which simply terminates experiments after they have run for a configured
amount.

Rejigs the monitoring class a bit to allow for per config type termination criteria

* 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.