feat(code/test): Middleware for testing

[romac]

Add a Middleware to the TestContext to override how votes and proposals are built, allowing us to change what a node votes or proposes on a per-test basis.

This was extracted from #896 to make it easier to review that PR.

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• Make new_prevote, new_precommit and new_proposal methods of Context instead of static functions

• Stuff middleware in the TestContext instead of static variable

• Remove unused dependency

• Add way to access middleware from TestContext

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PR author checklist

For all contributors

• Reference GitHub issue
• Ensure PR title follows the conventional commits spec

For external contributors

• Maintainers of Malachite are allowed to push changes to the branch

[romac]

@ancazamfir Nevermind my comments above (now deleted), I was wrong.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (3b291f7) to head (19893e4).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #948   +/-   ##
===========================
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Components	Coverage Δ
core	∅ <ø> (∅)
engine	∅ <ø> (∅)
app	∅ <ø> (∅)
starknet	∅ <ø> (∅)