E2E testing framework

[hairyhum]

Description

Addresses #1931 on the API side.

Adds the framework to run e2e tests using existing kargo instance with kargo API auth.

See README.md for more info.

Checklist

Eligibility

• Linked to an existing issue with no blocking labels (kind/proposal, needs discussion, needs research, maintainer only, area/security, size/large, size/x-large, size/xx-large).
• Changes documentation only.
• Changes ten lines or fewer.

Quality

• Adds or updates corresponding tests.
• Adds or updates corresponding documentation.

AI Use Disclosure

This PR was written:

• By a human without AI assistance.
• By a human with AI assistance. A human has reviewed every line prior to opening the PR.
• By an AI with human supervision. A human has reviewed every line prior to opening the PR.
• Entirely by an AI. No human has reviewed this prior to opening the PR.

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