feat: Added CameraComponent.withFixedResolution() constructor

[st-pasha]

Description

This PR adds new constructor for CameraComponent which simulates the old-style FixedResolutionViewport.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[spydon]

Why have a constructor for this instead of a real viewport class? Feels inconsistent that some viewports would be set as a CameraComponent constructor and that some set the viewport field.

[spydon]

@st-pasha do you think you'd be able to add a line in the docs about this method before we do the release today, so that we can include this one? :)