simulators/eth2: Extract Builder and Clients

[marioevz]

This PR extracts the following components into their own repository:
./simulators/eth2/common/builder -> https://github.com/marioevz/mock-builder
./simulators/eth2/common/clients -> https://github.com/marioevz/eth-clients

Reason is being able to use mock-builder as an external command, and this requires the structs and interfaces declared in eth-clients.

This results in removing lot of code from ./simulators/eth2/common to make it more reusable.

Other fixes included:

• Slot time configuration is now part of the common library and automatically configured for clients that don't support 6s slots
• Incremental forks are removed (trick to start from phase0 up to bellatrix/capella in order for some clients to work, no longer necessary)
• Fix TTD > 0 setup in eth2/engine

[marioevz]

Rebased to main to fix geth not working due to PoW.

[marioevz]

I'll finish up running the simulators with the change and merge in a few hours unless objections 👍 @fjl