op-batcher: testing API

[protolambda]

A gap between action-tests and full system tests is the op-batcher:
for some tests, we need control when the batch is wrapped-up and submitted.

After the op-test-sequencer work is done, a simple op-batcher API to influence the batch scheduler may be enough.

Currently the batcher determines when to submit batches based on the buffered data,
the time, and the sync state of the attached node.

With an RPC, we can ideally change what L2 blocks are in scope,
and override when the buffer is considered full enough to submit to L1.

E.g.:

• batcher_setL2Scope: limit the view over the L2 chain
• batcher_submitNow: force-submit the current data, even if the buffer is not full yet
• batcher_cursors: check where the cursor (submitted L2 block, different stages maybe) is at.

Tests can then use these to make the batcher submit txs when we want that.
The test-sequencer can then receive the tx from a regular eth_sendRawTransaction RPC, and confirm it in the right L1 block.