Refactor starter heuristics to be modular.

This CL defines a new way to specify starter heuristics. The old way
will be deprecated, but continue to be supported for a bit longer until
we can phase it out.

This is the first CL in a chain. This only adds the new system, but
does not (yet) replace the old one. See the downstream for the rest
of the refactor.

This CL also pre-allocated 5 default-disabled starter heuristic
features for use in future launches. This will allow us to deploy
multiple starter heuristics simultaneously, each with their own finch
rollout.

See the bug description for the full design proposal.

Bug: b/239682654
Change-Id: I22b2ec2bb53528e92125240776485943cb9a5fe8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3776660


Reviewed-by: Florian Gauger <fga@google.com>
Auto-Submit: Clemens Arbesser <arbesser@google.com>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/main@{#10269104}