interop: L1 read-only interop (exploratory)

[protolambda]

We can index the L1 chain as a read-only source of events.

This is not currently in scope, but may be explored as an extension of regular interop.

In the logsdb, the L1 chain would essentially be indexed just like a "L2 chain", and be allowed to emit initiating messages, but not execute any messages (as L1 forkchoice does not enforce interop inbox rules).

And the in the fromda DB of local/cross safe is relatively trivial here, since L1 blocks are dependent on the block itself and nothing else.