[BB-7904] fix: youtube race condition when GTM loads

[viadanna]

This PR fixes a race condition that might prevent YouTube videos from loading when Google Tag Manager is used.

Both Open edX and GTM implement onYouTubeIframeAPIReady and run already existing implementations of this callback when set up.

The issue arises when GTM overwrites it, as the callback implemented by Open edX expects a resolve() attribute that isn't implemented by GTM. This change ensures the original global is used by Open edX so it's never overwritten.

Testing

1. Set up a devstack with this branch and Google Analytics, or use this sandbox.
2. Navigate to a Unit containing a YouTube video and open your browser Development Tools.
3. Add the following breakpoint:
   
4. Reload the page and wait a couple seconds before pressing F8 to continue running. This ensures onYouTubeIframeAPIReady is overwritten by GTM.
5. Make sure the YouTube video loads correctly.
6. Disable the breakpoint and reload, again making sure the Youtube video loads when the callback set by Open edX is used.

[ArturGaspar]

👍

• I tested this: reproduced both the cases where GTM and Open edX set the callback first, tested that video works
• I read through the code
• I checked for accessibility issues - N/A
• Includes documentation - N/A
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository. - N/A

[0x29a]

Upstream PR: openedx#33649