[v2] Add blackbox tests

[aemous]

Notes:

• Merging this PR is currently a no-op. They can be run ad-hoc by developers but are not invoked in any automated CI.

Description of changes:

• Added scaffolding to easily add blackbox tests. The main helper function is assert_operation which lets callers verify if an HTTP request matches an operation. This lets test-writers easily verify if the expected operations were called in the expected order.
• Added S3 blackbox tests. These are ports of our existing S3 functional tests (except cp, mv, or sync commands). The only tests not ported are those that cannot be feasibly ported to a blackbox case (e.g. relies on a Python monkey-patch of datetime).
• Added new blackbox requirements test file, and updated regenerate-lock-files and GitHub workflows to account for this.

Description of tests:

• Ran and passed all new blackbox tests via PyTest.

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[hssyoo]

Looking good, mainly had some structural feedback.

[hssyoo]

Looks good, can we take a test dependency on localstub if we're going to be merging to v2