simulators/eth2: Common Library

[marioevz]

Coalesce common code between eth2 simulators into a library that allows code reuse.

[marioevz]

Updated to use latest geth version, which requires go.mod replace.

Changing to draft because merging would break other simulators until either:

• Withdrawals' changes are merged
• We use go.mod replace on the modules/simulators where the replace is breaking stuff.

[fjl]

These changes were merged as part of #681