[camera_web] Add createCamera implementation

[bselwe]

Adds createCamera implementation of the camera platform interface.

• Added mapFacingModeToCameraType to CameraSettings.
  • Maps the camera facing mode to a camera type, used when creating a camera.
• Added mapResolutionPresetToSize to CameraSettings.
  • Maps the resolution preset to a size, used when creating a camera.
  • Uses the same sizes as on Android.
• Added createCamera implementation.
  • Uses camera metadata initialized in availableCameras to extract the camera type.
  • Creates an uninitialized camera and constrains its audio, resolution, facing mode and device id with CameraOptions.
  • Returns the id of the created camera.

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! I'll revert the platform_interface change for now!

[ditman]

Should this take in to account the screen orientation? I don't remember if we had to flip the w/h on the photobooth when the app was rotated? Do you remember @felangel?

[bselwe]

Hmm, I can see that photobooth records the screen orientation when the photo is being captured. Then, the photo is displayed inside the AspectRatio widget with an aspect ratio based on that orientation but not sure if that has anything to do with flipping the w/h. 🤔