Audio mix with other apps for iOS

[euharrison]

This PR adds a feature to control how audio from video can mix with audios from others apps in iOS.

It's uses the options AVAudioSessionCategoryOptionMixWithOthers and AVAudioSessionCategoryOptionDuckOthers

It's helps #1103 and #959. It's also highly referenced by #403.

I'm not a iOS developer so please fells free to points to me any Obj-C error.

Sample:

<Video
  ...
  mixWithOthers="mix" // or "duck"
/>

[jenshandersson]

This looks good to me @euharrison, I can see how this could be useful. Thanks for submitting it! Is there a similar property for Android we can implement?

[jenshandersson]

I guess ignoreSilentSwitch is iOS only so then by that logic this can be added too :) Just make sure docs are clearly staging it's for iOS only.

[euharrison]

Thanks that you appreciate @jenshandersson :)

I added the Platforms: iOS in the README. If you suggest to add it in another file, please let me know
https://github.com/react-native-community/react-native-video/pull/1978/files#diff-04c6e90faac2675aa89e2176d2eec7d8R541

[jenshandersson]

Ah right, great. Can you add a line in the changelog as well @euharrison? Check other PRs to see how, basically a explanation and reference to PR

[euharrison]

Done ✅

[lousyfisherman07]

Jsusj