[image_picker] Add desktop support - platform interface

[stuartmorgan-g]

Platform interface portion of #3882

Adds CameraDelegatingImagePickerPlatform and ImagePickerCameraDelegate, and supportsImageSource

Part of flutter/flutter#102115
Part of flutter/flutter#102320
Part of flutter/flutter#85100

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[tarrinneal]

Just a question.

[tarrinneal]

Is this necessary?

[stuartmorgan-g]

Because Dart won't promote nullable fields (because in general it can't guarantee that you aren't running this code in the context of a subclass that overrode the field with a getter that does something crazy like return null every other time you call it), you have two options for cases like this:

• Make a local variable, null check it to promote it, and then use it as non-nullable.
• Null-check the field, then use ! every time you access it.

I strongly prefer the former since it relies on the compiler to ensure that things are still right any time someone changes the logic, while the latter relies on humans to ensure it.