Adds Apple Music icon to Social Icons Block

[morgan-on-foss]

What?

Why?

How?

Attempting to use the SVG file from Apple's files:

1. https://developer.apple.com/licensing-trademarks/
2. Apple Music Identity Guidelines: https://marketing.services.apple/apple-music-identity-guidelines
3. "3 Apple Music Icons" dropdown then "Download" button
4. Using the Apple_Music_Icon_RGB_sm_073120.svg file from Apple Music_Icon_2020.zip / Apple Music_Icon_2020 / AppleMusic_Icon_Color / RGB / SVG / Small
   • I opted for the RBG version of the SVG because I think CMYK is usually for print assets.

Expected:

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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[Mamaduka]

Closing in favor of #70261. The extensibility feature will ship in WP 6.9 and is already available in Gutenberg 21.1.

Here's an example plugin - https://github.com/wptrainingteam/devblog-custom-social-icons.