[camera_web] Add CameraOptions used to constrain the camera audio and video

[bselwe]

Adds CameraOptions, used to constrain the camera audio and video, to camera_web plugin.

These options represent web MediaStreamConstraints and can be used to request the browser for media streams with audio and video tracks containing the requested types of media.

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.)
• I signed the CLA.
• 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!