[Performance 2024] Added bytesPerPixel (BPP) property to Images custom metric

[kevinfarrugia]

Add bytesPerPixel custom metric. This is calculated as (filesize in bits) / (image display height * image display width)

Note that it would return 0 for cross-origin images that do not include the timing-allow-origin header.

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Test websites:

• https://web.dev
• https://almanac.httparchive.org/