Standard beacon api updates (#1831)

## Issue Addressed

Resolves #1809 
Resolves #1824
Resolves #1818
Resolves #1828 (hopefully)

## Proposed Changes

- add `validator_index` to the proposer duties endpoint
- add the ability to query for historical proposer duties
- `StateId` deserialization now fails with a 400 warp rejection
- add the `validator_balances` endpoint
- update the `aggregate_and_proofs` endpoint to accept an array
- updates the attester duties endpoint from a `GET` to a `POST`
- reduces the number of times we query for proposer duties from once per slot per validator to only once per slot 


Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/http_api/src/beacon_proposer_cache.rs

@@ -89,6 +89,7 @@ impl BeaconProposerCache {
 
                         Ok(ProposerData {
                             pubkey: PublicKeyBytes::from(pubkey),
+                            validator_index: i as u64,
                             slot,
                         })
                     })