Add custom beacon withdrawals example

[dapplion]

Adds an example of modifying a post-block execution step. Specifically, it modifies the beacon withdrawal processing logic: do a system call instead of minting native tokens.

I have made some structs and fns pub to reduce diff. Please let me know if there are more opportunities to minimize the diff. There's duplicated code between this custom executor, the op executor and the eth executor.

[gakonst]

feels like we should expose a better API via NodeBuilder for overriding the pre/post hooks more granularly, and for new system contracts, given BSC also uses it

[dapplion]

feels like we should expose a better API via NodeBuilder for overriding the pre/post hooks more granularly, and for new system contracts, given BSC also uses it

Let me know if I can help with that

[emhane]

lgtm

[dapplion]

@mattsse do you want to merge this in as-is, and have the satisfaction of reducing code once #11274 lands? Or wait to merge #11274 before this PR

[fgimenez]

hey @dapplion thanks a lot for your work! now that #11274 is closed I've taken the opportunity to rebase this PR on top of main and update it to use the new composable executor abstraction with strategies.

In the BlockExecutionStrategy impl I have filled the pre and post block execution methods same as you had before, but not the actual block transaction execution logic method, not sure if that's ok for the functionality you want to highlight.

pls take a look and let me know if you have any questions!

[Rjected]

cool! lgtm

[dapplion]

🎉 thanks for updating it to the custom executor