[builder API] Add --builder-timestamp-override

[realbigsean]

Description

From @ciaranmcveigh in Discord

for distributed validators it's important that the signed contents are the same across all "co-validators", time.now is likely to never match across multiple validators (the resolution on other validator clients so far has been to include a -builder-timestamp-override flag that enables the user to hardcode a value, this value is then used for every subsequent re-registration, to update the the registration config the co validators need to communicate the changes and reboot the system with the new timestamp)

[realbigsean]

Resolved in #3340