feat: Add a new whenAppLayoutReady to the plugin api

[georgylobko]

Description

Added a new whenAppLayoutReady to the plugin api to keep track of AppLayout state loading. It's useful when left drawer needs to be updated - in this case making sure that AppLayout has already loaded is necessary.

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.22%. Comparing base (d1c8b7f) to head (c8d86f2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3863   +/-   ##
=======================================
  Coverage   97.22%   97.22%           
=======================================
  Files         844      844           
  Lines       24586    24595    +9     
  Branches     8672     8677    +5     
=======================================
+ Hits        23904    23913    +9     
+ Misses        675      633   -42     
- Partials        7       49   +42     

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[just-boris]

1. This code does not work, because the remove widget and the app layout widget are separate bundles, so they will refer to a different appLayoutReadyDefer instance

2. What is going to happen if app layout renders and then unmounts? This code resolves the promise instantly, but there is no app layout on the page

3. What happens if app layout never renders? The promise will hang forever in this case. We should mention to the consuming team that they might want to add some timeout on their end, for example: Promise.race([whenAppLayoutReady(), timeout(3000)])

[georgylobko]

1. Addressed. Now this promise is stored in window object, so any instance will have access to it
2. The purpose of this promise is to be resolved as soon as the app layout is available, it's not designed to handle other use cases such as unmounting. Do you have any suggestions on that?
3. Point taken

[just-boris]

2. Reverse the control.

export function whenAppLayoutReady() {
   if(isAppLayoutReady()) {
        return Promise.resolve();
   }
   return new Promise((resolve) => appLayoutReadyCallbacks.push(resolve));
}

function registerAppLayoutHandler () {
   appLayoutReadyCallbacks.forEach(() => cb());
   appLayoutReadyCallbacks = [];

    // other code...
}

[just-boris]

See this thread #3863 (comment)

[just-boris]

You do not need to introduce the defer helper any more. The code from my previous comment works without it