Validators API needs filtering

When calling `/eth/v1/beacon/states/head/validators?status=active` over 1 gigabyte of data is returned. We need to be able to get only the fields requested for various reasons. Such as only the indexes of the active validators which would reduce the size of the data returned significantly (~4 KB).

Proposal:

Allow filtering by fields, such as `/eth/v1/beacon/states/head/validators?status=active&fields=index,public_key`

Start with the `/eth/v1/beacon/states/head/validators` endpoint definition and review how its existing `status` query parameter is specified. Define the requested `fields=index,public_key` filtering behavior and confirm that the specification clearly describes the response for selected fields and preserves existing behavior when no fields are requested.

Written by the indexing model from the issue text.