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/http_api/src/version.rs

@@ -1,4 +1,6 @@
-use crate::api_types::{EndpointVersion, ForkVersionedResponse};
+use crate::api_types::{
+    EndpointVersion, ExecutionOptimisticForkVersionedResponse, ForkVersionedResponse,
+};
 use eth2::CONSENSUS_VERSION_HEADER;
 use serde::Serialize;
 use types::{ForkName, InconsistentFork};
@@ -25,6 +27,26 @@ pub fn fork_versioned_response<T: Serialize>(
     })
 }
 
+pub fn execution_optimistic_fork_versioned_response<T: Serialize>(
+    endpoint_version: EndpointVersion,
+    fork_name: ForkName,
+    execution_optimistic: bool,
+    data: T,
+) -> Result<ExecutionOptimisticForkVersionedResponse<T>, warp::reject::Rejection> {
+    let fork_name = if endpoint_version == V1 {
+        None
+    } else if endpoint_version == V2 {
+        Some(fork_name)
+    } else {
+        return Err(unsupported_version_rejection(endpoint_version));
+    };
+    Ok(ExecutionOptimisticForkVersionedResponse {
+        version: fork_name,
+        execution_optimistic: Some(execution_optimistic),
+        data,
+    })
+}
+
 /// Add the `Eth-Consensus-Version` header to a response.
 pub fn add_consensus_version_header<T: Reply>(reply: T, fork_name: ForkName) -> WithHeader<T> {
     reply::with_header(reply, CONSENSUS_VERSION_HEADER, fork_name.to_string())