interop: test-sequencing milestone 1: L1 mode

[protolambda]

See https://github.com/ethereum-optimism/design-docs/blob/main/protocol/test-sequencing.md#milestone-1-l1-mode

Minimal version:

• No L1 CL node
• Single L1 EL node
• L2 nodes all connected to this EL
• Make the test-sequencer

In op-e2e we have a "fake beacon node", that serves a tiny subset of the beacon API to make blobs available to L2 nodes, while basically looping the engine-API instructions to make EL blocks, without any real beacon consensus.

We can support this type of block-building by:

• implementing a builder package that connects to engine-API directly, and builds some simple native ethereum blocks (no payload attributes to prepare like in L2)
• implementing a block type that fits the block interface of the test-sequencer, such that the regular full-sequencer package can loop over this, to build the mock L1 chain like we would with a L2 chain.