refactor: use ImageHandler from @netlify/images

[keanu-a]

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Summary

Refactors logic of the initializeImageProxy function to use @netlify/images instead of express function. Removed unit tests for the old logic as well.

Fixes #EX-536

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📊 Benchmark results

Comparing with 99a2f05

• Dependency count: 1,077 ⬆️ 0.09% increase vs. 99a2f05
• Package size: 297 MB ⬆️ 0.01% increase vs. 99a2f05
• Number of ts-expect-error directives: 382 (no change)