Consider supporting using ethers-middleware's TimeLag rather than ad-hoc solutions

[nyospe]

See e.g. latest_confirmed_block_number https://github.com/EspressoSystems/cape/blob/main/eqs/src/configuration.rs#L27

It would be better to explicitly make the connection's Middleware assume the correct lag, and make optimistic calls against that connection explicit.

[sveitser]

Thanks for finding this.