Add a flag for storing invalid blocks (#4194)

## Issue Addressed

NA

## Proposed Changes

Adds a flag to store invalid blocks on disk for teh debugz. Only *some* invalid blocks are stored, those which:

- Were received via gossip (rather than RPC, for instance)
    - This keeps things simple to start with and should capture most blocks.
- Passed gossip verification
    - This reduces the ability for random people to fill up our disk. A proposer signature is required to write something to disk.

## Additional Info

It's possible that we'll store blocks that aren't necessarily invalid, but we had an internal error during verification. Those blocks seem like they might be useful sometimes.

beacon_node/network/src/beacon_processor/tests.rs

@@ -203,6 +203,7 @@ impl TestRig {
             max_workers: cmp::max(1, num_cpus::get()),
             current_workers: 0,
             importing_blocks: duplicate_cache.clone(),
+            invalid_block_storage: InvalidBlockStorage::Disabled,
             log: log.clone(),
         }
         .spawn_manager(beacon_processor_rx, Some(work_journal_tx));