Implement POST get validators endpoint

[xenowits]

🎯 Problem to be solved

A new endpoint has been recently added to the beacon APIs to query validators from the beacon chain

POST /eth/v1/beacon/states/{state_id}/validators

Note that we already have an endpoint for the same purpose of querying validators, but it's a GET that takes in validator pubkeys/indices in a query parameter

GET /eth/v1/beacon/states/{state_id}/validators

🛠️ Proposed solution

Implement the new POST endpoint in core/validatorapi.

This will probably require us to read the bytes in the request body (router.go):

...
func getValidators(p eth2client.ValidatorsProvider) handlerFunc {
	return func(ctx context.Context, params map[string]string, query url.Values, _ contentType, _ []byte) (any, http.Header, error) {
		stateID := params["state_id"]

		resp, err := getValidatorsByID(ctx, p, stateID, getValidatorIDs(query)...)
...

So, one approach might be to:

1. Read query parameters for validator pubkeys/indices
2. If no query parameters provided, read request body
3. If query parameters provided, don't read request body

🧪 Tests

• Tested by new automated unit/integration/smoke tests
• Manually tested on core team/canary/test clusters
• Manually tested on local compose simnet

[dB2510]

Hey team! Please add your planning poker estimate with Zenhub @dB2510 @gsora @pinebit @xenowits

[pinebit]

Tested