[Feature/extensions] Meta: Merge back to main tracking issue

[ryanbogan]

This is a tracking issue for merging the feature/extensions branch into main, and then backported to 2.x.

The merge will be performed in four steps, and all functionality will be behind a feature flag.

Meta-issue for experimental release: 139

Sub-issues:

• [Feature/extensions] Merge to main tracking - Initial extensions framework #5264
• [Feature/extensions] Merge to main tracking - Extension Points, initial REST implementation and registering Transport Actions #5271
• [Feature/extensions] Merge to main tracking - getSettings() support, ActionListener onFailure(), and initial createComponents support #5272
• [Feature/extensions] Merge to main tracking - REST, TranportActions, and EnvironmentSettings updates (up to present) #5273