[Gutenberg] Audio upload processor

[etoledom]

gutenberg PR: WordPress/gutenberg#27401
gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#2854

This PR implements the Audio upload processor to be used together with the Audio block.

This PR also improves the file type filet for the on-device file picker: Now only file extensions allowed by the blog can be picked. Previously any video/image extensions could be picked for the Audio/Video blocks. This is also now used on the Audio block to avoid Apple-only audio filetypes (common on audio files recorded in iPhones).

To test:
Test running metro server on wordpress-mobile/gutenberg-mobile#2854

• On a new post, add an Audio block.
• Add to it a relatively large Audio file.
• While the upload is happening, close the editor saving as a draft.
  • The post card will show an upload in progress.
• Wait for the upload to finish and the post to be saved remotely.
• Open the saved post.
• Switch to HTML mode.
  • Check that the Audio block id has a positive number.
  • Check that the src URL is a remote (non-local) URL.

cc @jd-alexander

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[guarani]

Looks good to me @etoledom! Left a few minor code comments.

[etoledom]

Thank you @guarani 🙏
Nice catch! Committed the suggestions 👍