Add benchmark flashblock websocket server

[meyer9]

Add a method SupportsFlashblocks() to the validator client (or reuse existing flag if it's available?).

If the client supports flashblocks, before running the client, set up a flashblock server that will emit flashblock payloads that the client will read.

The websocket URL will be passed to the client upon starting, so it needs to be started before that.

Flashblocks should be sent in evenly spaced intervals to the client mimicing how the client would receive the flashblocks from the sequencer.

Flashblock payloads are already collected from the sequencer in this PR: f95a703

These should be replayed to test flashblock validation latency.