perf(sequencer): add benchmark for prepare_proposal (ENG-660)

[Fraser999]

Summary

Addition of a new benchmark target to assess the performance of the prepare_proposal method in sequencer's App.

Background

Previous perf work has indicated this is a bottleneck. However, making that determination was done via spamoor which is slightly convoluted to run. Before working to improve the performance, we want to have a faster feedback loop on the effects of updates, hence the need for a benchmark which is easy to run and which isolates the slow function.

Changes

• Added benchmark to app module. Currently this has only one case: a mempool filled with transactions containing exclusively transfers. This matches the shape of the data being sent when using spamoor.
• Added benchmark feature to enable sharing some of the existing test utils.

Testing

This is a new test.

Example of running cargo bench --features=benchmark -qp astria-sequencer app on my Ryzen 7900X:

Timer precision: 10 ns
astria_sequencer                                fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ app                                                        │               │               │               │         │
   ╰─ benchmarks                                              │               │               │               │         │
      ╰─ execute_transactions_prepare_proposal  11.63 s       │ 14.68 s       │ 12.74 s       │ 12.88 s       │ 10      │ 10

Since rebasing after #1317 has merged, the same run shows (as expected) a slowdown:

astria_sequencer                                fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ app                                                        │               │               │               │         │
   ╰─ benchmarks                                              │               │               │               │         │
      ╰─ execute_transactions_prepare_proposal  14.49 s       │ 17 s          │ 16.52 s       │ 15.98 s       │ 8       │ 8

Related Issues

Closes #1314.

[Fraser999]

Closed in favour of #1337 (this PR was cut from the wrong repo).