Add execution_optimistic flag to HTTP responses (#3070)

## Issue Addressed

#3031 

## Proposed Changes

Updates the following API endpoints to conform with https://github.com/ethereum/beacon-APIs/pull/190 and https://github.com/ethereum/beacon-APIs/pull/196
- [x] `beacon/states/{state_id}/root` 
- [x] `beacon/states/{state_id}/fork`
- [x] `beacon/states/{state_id}/finality_checkpoints`
- [x] `beacon/states/{state_id}/validators`
- [x] `beacon/states/{state_id}/validators/{validator_id}`
- [x] `beacon/states/{state_id}/validator_balances`
- [x] `beacon/states/{state_id}/committees`
- [x] `beacon/states/{state_id}/sync_committees`
- [x] `beacon/headers`
- [x] `beacon/headers/{block_id}`
- [x] `beacon/blocks/{block_id}`
- [x] `beacon/blocks/{block_id}/root`
- [x] `beacon/blocks/{block_id}/attestations`
- [x] `debug/beacon/states/{state_id}`
- [x] `debug/beacon/heads`
- [x] `validator/duties/attester/{epoch}`
- [x] `validator/duties/proposer/{epoch}`
- [x] `validator/duties/sync/{epoch}`

Updates the following Server-Sent Events:
- [x]  `events?topics=head`
- [x]  `events?topics=block`
- [x]  `events?topics=finalized_checkpoint`
- [x]  `events?topics=chain_reorg`

## Backwards Incompatible
There is a very minor breaking change with the way the API now handles requests to `beacon/blocks/{block_id}/root` and `beacon/states/{state_id}/root` when `block_id` or `state_id` is the `Root` variant of `BlockId` and `StateId` respectively.

Previously a request to a non-existent root would simply echo the root back to the requester:
```
curl "http://localhost:5052/eth/v1/beacon/states/0xaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/root"
{"data":{"root":"0xaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}}
```
Now it will return a `404`:
```
curl "http://localhost:5052/eth/v1/beacon/blocks/0xaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/root"
{"code":404,"message":"NOT_FOUND: beacon block with root 0xaaaa…aaaa","stacktraces":[]}
```

In addition to this is the block root `0x0000000000000000000000000000000000000000000000000000000000000000` previously would return the genesis block. It will now return a `404`:
```
curl "http://localhost:5052/eth/v1/beacon/blocks/0x0000000000000000000000000000000000000000000000000000000000000000"
{"code":404,"message":"NOT_FOUND: beacon block with root 0x0000…0000","stacktraces":[]}
```

## Additional Info
- `execution_optimistic` is always set, and will return `false` pre-Bellatrix. I am also open to the idea of doing something like `#[serde(skip_serializing_if = "Option::is_none")]`.
- The value of `execution_optimistic` is set to `false` where possible. Any computation that is reliant on the `head` will simply use the `ExecutionStatus` of the head (unless the head block is pre-Bellatrix).

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

beacon_node/beacon_chain/src/test_utils.rs

@@ -37,6 +37,7 @@ use state_processing::{
 };
 use std::borrow::Cow;
 use std::collections::{HashMap, HashSet};
+use std::fmt;
 use std::str::FromStr;
 use std::sync::Arc;
 use std::time::Duration;
@@ -1778,3 +1779,10 @@ where
         (honest_head, faulty_head)
     }
 }
+
+// Junk `Debug` impl to satistfy certain trait bounds during testing.
+impl<T: BeaconChainTypes> fmt::Debug for BeaconChainHarness<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "BeaconChainHarness")
+    }
+}