[Incomplete] Add icons for popular podcast services

[properlypurple]

What?

This PR aims to add icons for some popular podcast services to the social icons block.

Why?

The social icons block doesn't have icons for some popular podcasting services, and doesn't have a straightforward way to add custom icons. This PR fixes #36755

How?

I aim to add the following icons

• Apple podcasts
• Google podcasts
• Pocketcasts
• Pandora
• TuneIn

I have only added Google and Pocketcasts for now. There are significant differences in the brand guidelines for these services, and this will likely need design feedback before going any further. For reference

• Apple
  • https://www.apple.com/itunes/marketing-on-podcasts/identity-guidelines.html#apple-podcasts-icon
  • No option for single-color icon except b/w, or a separate background
• Google
  • https://support.google.com/podcast-publishers/answer/9798635?hl=en&sjid=3110168449627324902-AP
  • https://about.google/brand-resource-center/rules/
  • No mention of just the logo, colors, etc
• TuneIn
  • https://cms.tunein.com/brand/
• Pocketcasts
  • https://pocketcasts.com/podcast-producers/
  • No mention of simple logo
• Pandora
  • https://developer.pandora.com/docs/developer-resources/brand-assets/

Testing Instructions

1. Open a post or page to edit.
2. Add a social icons block.
3. Try adding icons for the services mentioned above.
4. Try changing colors and/or icon style from the block settings.

Testing Instructions for Keyboard

This doesn't change existing interactions for the social icons block, so the usual steps for this block apply.

Screenshots or screencast

[github-actions]

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[properlypurple]

Closing because this is likely going to be redundant with #30749