fix: consistent behavior with multiple invocations of SwaggerUI

[shockey]

Description

This PR fixes a bug in which Swagger UI's system constructor will, with certain plugin structures, mangle the original plugin that was passed into Swagger UI.

This leads to inconsistent behavior when the same object is passed into Swagger UI again, which is part of some users' workflows.

Motivation and Context

Fixes #4865.

How Has This Been Tested?

I added Cypress tests for the top-level behavior mentioned in the ticket, and unit tests for the underlying system mechanism.

Per TDD, both sets of tests fail without this change.

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.