feat: allow custom image scales on iOS

[edusperoni]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

Images are always loaded with scale 1.0 (if they are files/resources then they're loaded with the appropriate scale when @2x.png is provided, for example)

What is the new behavior?

Allows you to set a hardcoded scale for the image.

[nx-cloud]

☁️ Nx Cloud Report

CI is running/has finished running commands for commit af106de. As they complete they will appear below. Click to see the status, the terminal output, and the build insights.

📂 See all runs for this branch

---

✅ Successfully ran 2 targets

• nx run-many --target=test --configuration=ci --projects=core
• nx run webpack5:build

---

Sent with 💌 from NxCloud.

[edusperoni]

This might be too much of a burden to support in the future (especially if we decide to support orientation as well).

This is a viable alternative that's around 0.02ms slower and doesn't require any code to support:

function setImageSourceScale(source: ImageSource, scale: number) {
  const image = source.ios;
  return new ImageSource(UIImage.imageWithCGImageScaleOrientation(image.CGImage, scale, image.imageOrientation));
}