Don't download ext_index.json from s3, but instead receive it as a part of spec from control plane.
This eliminates s3 access for most compute starts,
and also allows us to update extensions spec on the fly
This is in preparation of using compute_ctl to launch postgres nodes
in the neon_local control plane. And seems like a good idea to
separate the public interfaces anyway.
One non-mechanical change here is that the 'metrics' field is moved
under the Mutex, instead of using atomics. We were not using atomics
for performance but for convenience here, and it seems more clear to
not use atomics in the model for the HTTP response type.
