[camera_web] Add Camera used to control video and take pictures

[bselwe]

Adds Camera to camera_web plugin. It is used to play/stop the video stream and take a picture. May be constrained with CameraOptions.

Part of flutter/flutter#45297

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. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
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• The title of the PR starts with the name of the plugin 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.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[bselwe]

@ditman @felangel @orestesgaolin

[ditman]

LGTM! I'm guessing by the direction that this is going, that soon you'll implement a class that extends the CameraPlatform and uses this Camera object?

[bselwe]

Exactly, the next step is implementing availableCameras and createCamera that creates this Camera object.