Add ImageExtractor for renderer

This commit introduces a new `ImageExtractor` class in the content
renderer process for the wallet component. This class is responsible for
extracting images from the main frame of a webpage.

The `ImageExtractor` exposes a Mojo interface for browser-renderer
communication. Its lifetime is tied to the `RenderFrame` by being stored
as `UserData`. It can be used to request all images from the document,
which are returned as a vector of `SkBitmap` objects.

Browser tests are included to verify the image extraction logic.

This CL is based on the work of minjiaw@google.com in Gerrit change:
https://chromium-review.googlesource.com/c/chromium/src/+/6845376

Bug: 438396005
Change-Id: Iae037f986063a47c685d84c93348ffae04f69000
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6916644
Reviewed-by: Norge Vizcay <vizcay@google.com>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Dongguo Zhang <dongguo@google.com>
Reviewed-by: Bruno Braga <brunobraga@google.com>
Cr-Commit-Position: refs/heads/main@{#1514717}