chore: Improve the iOS CI job

[jbelkins]

Description of changes

Replaces the iOS Github CI job with a new job having the following improvements:

• Does not use the CRT Builder script. Provides its own lightweight scripting for project setup.
• No dependency on access to any AWS account or any special Github credentials.
• No need to maintain any external build container or other resource.
• Builds protocol tests & unit tests together, reducing build time by de-duplicating several build steps.
• Uses xcodebuild to build & test the SDK using the iOS Simulator.
• xcbeautify is used to clean up & reduce the volume of Swift logs, and to reduce build time.

Once this job has had an opportunity to run for a while and proves reliable & efficient, I plan to adapt it for use with other CI jobs as well.

New/existing dependencies impact assessment, if applicable

No new dependencies were added to this change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[waahm7]

Why set HOMEBREW_NO_AUTO_UPDATE? Each CI run will be re-installing xcbeautify.

[jbelkins]

Homebrew will automatically update whenever it's invoked, unless this var is set.

The Github CI machines are often pretty far behind on the Homebrew repo, so updating homebrew takes a fair amount of time.

xcbeautify itself doesn't get updated very often, and I'm OK with being behind the latest version.