[camera_web] Add support for a zoom level

[bselwe]

Adds support for a zoom level to the web camera platform.

• Added getZoomLevelCapabilityForCamera to CameraSettings.
  • Returns the zoom level capability for the given camera. Includes the minimum and maximum zoom levels and the video track capable of configuring the zoom level.
  • Throws a CameraWebException if the zoom level is not supported or the camera has not been initialized or started.
• Added getMaxZoomLevel, getMinZoomLevel and setZoomLevel to Camera and CameraPlugin.
  • Each method uses CameraSettings.getZoomLevelCapabilityForCamera to access the camera zoom level capability.
  • setZoomLevel throws a CameraWebException if the zoom level is invalid (not in the valid range).

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

[ditman]

Nice!