feat(bench): generate traffic calling MPC contract's sign method

[mooori]

Requirements have been discussed in this Zulip thread.

Usage

Assumes the contract has already been deployed and initialized.

1. Create accounts that send transactions, e.g. with just create_sub_accounts.
2. Execute the benchmark-mpc-sign command.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.49%. Comparing base (f06f798) to head (ed6e0c5).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12658      +/-   ##
==========================================
- Coverage   70.50%   70.49%   -0.01%     
==========================================
  Files         845      845              
  Lines      172261   172258       -3     
  Branches   172261   172258       -3     
==========================================
- Hits       121454   121436      -18     
- Misses      45712    45726      +14     
- Partials     5095     5096       +1     

Flag	Coverage Δ
backward-compatibility	0.16% <ø> (ø)
db-migration	0.16% <ø> (ø)
genesis-check	1.36% <ø> (+<0.01%)	⬆️
linux	69.36% <ø> (-0.01%)	⬇️
linux-nightly	70.09% <ø> (-0.01%)	⬇️
pytests	1.66% <ø> (+<0.01%)	⬆️
sanity-checks	1.47% <ø> (+<0.01%)	⬆️
unittests	70.32% <ø> (-0.01%)	⬇️
upgradability	0.20% <ø> (ø)

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