[trivial] Add builder submission offset from end of slot flag and set default to 3 seconds from end of slot

[Wazzymandias]

📝 Summary

• Based on data analysis, we find that validators make a getHeader call near the end of the slot, see: reference#1

• The activity of other builders indicates submissions coalesce around t-3 to t+1, where t is the end of the slot, see: reference#2

• Currently, Flashbots builder submits starting at t-4.

  • This was always meant to be a temporary measure to make sure we don't risk the stability of existing builder by inducing too late a submission.

• This task adds support for changing the submission to t-3 and exposes configurable CLI and environment variable parameters for builder submission offset.

    --builder.submission_offset value (default: 3s)
          Determines the offset from the end of slot time that the builder will submit
          blocks. For example, if a slot is 12 seconds long, and the offset is 2 seconds,
          the builder will submit blocks at 10 seconds into the slot.
          [$FLASHBOTS_BUILDER_SUBMISSION_OFFSET]

📚 References

Validator getHeader

Builder Bid Time Range

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• I have seen and agree to CONTRIBUTING.md