feat: onComplete config option

[shockey]

Description

This PR introduces an onComplete config option that fires one tick after updateJsonSpec is called following an updateSpec call.

In practice, this option should serve as a safe place to put code that touches Swagger-UI's DOM (though we continue encouraging users to instead use the plugin system to modify the UI).

Motivation and Context

Fixes #3028.

How Has This Been Tested?

This change can not be easily unit tested. I've added integration tests that will become when #4090 is merged.

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.