Support for Keystore v4 (EIP-2335)

[naps62]

Is your feature request related to a problem? Please describe.

ethers currently does not support the new EIP-2335 Keystore format
This lack of support actually comes from https://github.com/roynalnaruto/eth-keystore-rs, and not directly the ethers codebase, but still worth documenting and tracking work

EIP-2335 signatures are already used live in, at least, Rocket Pool setups. So this makes it impossible to create CLI automations that run on top of existing Rocket Pool configs

(I'm sure there are many more examples. this is just the one I'm personally familiar with)

Describe the solution you'd like

Support for EIP-2335. Ideally in a seamless way. as an ether's user, I shouldn't need to be aware of which keystore version I'm targeting. it should just work with the existing API)

Describe alternatives you've considered

Currently pending discussion here: roynalnaruto/eth-keystore-rs#18
considering another approach where v3/v4 are implemented as separate modules, perhaps through a feature flag to choose which one. This would probably imply ethers-rs users would have to choose which flag to compile for, or explicitly use two different APIs for each version

IMHO, not ideal, but I may not be familiar with all the technicalities

Additional context

I already started work on this here: roynalnaruto/eth-keystore-rs#18 (actually adapted from a previous PR, can't claim credit for that initial work)
Currently pending feedback from @roynalnaruto regarding some design decisions