Define a way to apply "carry patches" on top of the current base version of libevm

[darioush]

Rationale

It's possible that some modifications (Esp bug fixes) should be applied before we can bump the libevm base version.
This could be because the patch is not accepted in master yet, was rejected from upstream, or has been accepted in upstream but libevm has not updated the base version yet.

It should be clear when the patch can be removed, etc.

Implementation

For a first one, this fixes some ptr referencing in the chain config compatibility error, which is relied upon in subnet-evm tests ethereum#29514

[ARR4N]

I've assigned to myself to write up the release-branch approach that you suggested.