[Storefront Preview 🔍 ] Add "onContextChange" Property to StorefrontPreview Component

[bendvc]

Description

This is the companion PR for #466 in the Runtime-Admin project.

The goal in this PR is the expose the context changed message event in the form of a handler. This will allow StorefrontPreview implementers to define this handler and take the context and do what they want with it. The use case we are looking at is to integrate 3rd party CMS's like Amplience.

Types of Changes

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Breaking change (could cause existing functionality to not work as expected)
• Other changes (non-breaking changes that does not fit any of the above)

Changes

• Add onContextChange prop to StorefrontPreview component.
• Move custom prop validator to utilities

How to Test-Drive This PR

• Visit this preview link
• The page will load, and after 7.5s you should see the data reload navigation happen.
• This happens after 7.5 seconds because the storefront defined a 7.5 second sleep in the form of an async onContextChange handler.
• You can repeat this by clicking the "Preview" button, the button will immediately grey out, but the page will only load after 7.5s

Checklists

General

• Changes are covered by test cases
• CHANGELOG.md updated with a short description of changes (not required for documentation updates)

Accessibility Compliance

You must check off all items in one of the follow two lists:

• There are no changes to UI

or...

• Changes were tested with a Screen Reader (iOS VoiceOver or Android Talkback) and had no issues
• Changes comply with WCAG 2.0 guidelines levels A and AA
• Changes to common UI patterns and interactions comply with WAI-ARIA best practices

Localization

• Changes include a UI text update in the Retail React App (which requires translation)