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 70.72%. Comparing base (abe4a60) to head (1b874a9).
Report is 2 commits behind head on master.

Additional details and impacted files

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##           master   #12658      +/-   ##
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- Misses      45857    45883      +26     
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db-migration	0.16% <ø> (ø)
genesis-check	1.35% <ø> (ø)
linux	69.23% <ø> (+0.02%)	⬆️
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pytests	1.65% <ø> (ø)
sanity-checks	1.46% <ø> (ø)
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upgradability	0.20% <ø> (ø)

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[mooori]

Tests against v1.signer-dev.testnet have shown that the benchmark-mpc-sign command added here does its job. When running with num_accounts close to transactions_per_second, nonce conflicts can occourr on testnet [ref]. I suggest to improve the handling of that in a separate PR. For now the workaround is to use a number of accounts sufficiently larger than TPS, for instance 50 accounts for 30 TPS.

[mooori]

@saketh-are is 16 a reasonable value for the path length? In case the required path length may change, it can be made an argument (like --key-version above).