Implement POST validators/validator_balances APIs

[michaelsproul]

Issue Addressed

Some API users want to fetch large chunks of the validator set, and currently do so using the id param in the query string. However, query strings are usually subject to length limits on both the client and server side. For Lighthouse, we can't extend the length limit without forking our HTTP server (see #4333 (comment)).

Proposed Changes

Bypass the issue of query string limits by adding a POST variant of the /eth/v1/beacon/states/{state_id}/validators method. It takes the exact same parameters in a JSON body that looks like this:

{
  "ids": ["1"],
  "statuses": ["active"]
}

Update (Dec 2023): this is part of the beacon API spec now.

Additional Info

I would like the RocketPool analytics tools to use this, as they're currently using individual validator queries and large batches in parallel, which frustrate Lighthouse's state load mechanisms. Even with tree-states, large numbers of parallel queries are hard to deal with, because we have to load the entire state, even if only one validator is required. We have caches, but these typically only take effect after one of the requests completes. In practice this is several thousands times slower than if the end-user just loaded the whole validator set once. However users are (rightly) reluctant to do this, because the size of the JSON response is unwieldy. The best of both worlds is to get all the desired validators (and nothing else) in one request: i.e. using a POST request enabled by this PR.

[paulhauner]

Nice, simple and valuable!