Add support for cropping pictures in proxy camera

[glpatcern]

This PR is motivated by having some public panoramic webcams in my HA setup: those cameras have a large width/height ratio and their preview in HA is far from optimal. A cropped version of the image gives a better preview and the full picture is still accessible by clicking on it, as per default behavior of the proxy camera. I think this can be useful in general, not only for this case.

The PR includes extending the configuration to introduce a mode (either resize, default, or crop) and further parameters for the crop operation.

The crop mode is supported both on static images and on video streams, but I could only test it on a static image. I'm happy to do further testing on an MJPEG stream if I can get one.

Edit: this was successfully tested on a video stream, though with some caveats as illustrated in #39924.

Description:

Related issue (if applicable): N/A

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#7491

Example entry for configuration.yaml (if applicable):

camera:
  - platform: proxy
    name: My cropped view
    entity_id: camera.original
    mode: crop
    image_left: 800
    max_image_width: 1100

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io.

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
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If the code does not interact with devices:

• Tests have been added to verify that the new code works.