Add Sepolia PoW chain support

[jochem-brouwer]

One of the questions at the ArrowGlacier PR was (see #1527): why is there no difficulty bomb delay planned for Ropsten? Well, this is because the testnet will be deprecated and replaced by Sepolia. See ethereum/go-ethereum#23730

It seems that this will replace Ropsten after the merge, so it has no high priority, but it is anyways nice to implement this at some point and should not be a lot of work.

CC @holgerd77

[holgerd77]

Ah, oh, that's nice, have a new on-the-beginning lightweight PoW testnet at hand soon. 🙂

[holgerd77]

Ah, just remember that this was also discussed along the Merge even, lol (oh oh, the memories). 😜

[emersonmacro]

@holgerd77 @jochem-brouwer I took a stab at this here #1581. I haven't set up a new chain before so there might be something obvious that I missed - any feedback welcome. Specifically in chains/sepolia.json, I couldn't find anything for url or dnsNetworks, and not sure if I got the hardforks right.

[holgerd77]

Closed by #1581