Disallow attesting to optimistic head (#3140)

## Issue Addressed

NA

## Proposed Changes

Disallow the production of attestations and retrieval of unaggregated attestations when they reference an optimistic head. Add tests to this end.

I also moved `BeaconChain::produce_unaggregated_attestation_for_block` to the `BeaconChainHarness`. It was only being used during tests, so it's nice to stop pretending it's production code. I also needed something that could produce attestations to optimistic blocks in order to simulate scenarios where the justified checkpoint is determined invalid (if no one would attest to an optimistic block, we could never justify it and then flip it to invalid).

## Additional Info

- ~~Blocked on #3126~~

consensus/proto_array/src/proto_array.rs

@@ -387,7 +387,7 @@ impl ProtoArray {
                 ExecutionStatus::Irrelevant(_) => return Ok(()),
                 // The block has an unknown status, set it to valid since any ancestor of a valid
                 // payload can be considered valid.
-                ExecutionStatus::Unknown(payload_block_hash) => {
+                ExecutionStatus::Optimistic(payload_block_hash) => {
                     node.execution_status = ExecutionStatus::Valid(payload_block_hash);
                     if let Some(parent_index) = node.parent {
                         parent_index
@@ -458,7 +458,7 @@ impl ProtoArray {
             match node.execution_status {
                 ExecutionStatus::Valid(hash)
                 | ExecutionStatus::Invalid(hash)
-                | ExecutionStatus::Unknown(hash) => {
+                | ExecutionStatus::Optimistic(hash) => {
                     // If we're no longer processing the `head_block_root` and the last valid
                     // ancestor is unknown, exit this loop and proceed to invalidate and
                     // descendants of `head_block_root`/`latest_valid_ancestor_root`.
@@ -516,7 +516,7 @@ impl ProtoArray {
                             payload_block_hash: *hash,
                         })
                     }
-                    ExecutionStatus::Unknown(hash) => {
+                    ExecutionStatus::Optimistic(hash) => {
                         invalidated_indices.insert(index);
                         node.execution_status = ExecutionStatus::Invalid(*hash);
 
@@ -580,7 +580,7 @@ impl ProtoArray {
                                 payload_block_hash: *hash,
                             })
                         }
-                        ExecutionStatus::Unknown(hash) | ExecutionStatus::Invalid(hash) => {
+                        ExecutionStatus::Optimistic(hash) | ExecutionStatus::Invalid(hash) => {
                             node.execution_status = ExecutionStatus::Invalid(*hash)
                         }
                         ExecutionStatus::Irrelevant(_) => {