[Embed block] Include Jetpack embed variants

[jd-alexander]

Description

Jetpack has embed variants for some of the providers we support such as Facebook. The embed block should support all embed variants in Gutenberg and Jetpack.

• Embed variants index
• Facebook variant

We would have to refactor how we integrate blocks from Jetpack because these variations overwrite the core ones (reference). This means that we would need to import the Jetpack's embed variations before registering the core blocks for this purpose.

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Issue inspired by the discussion on this PR WordPress/gutenberg#34626

[fluiddot]

Heads up that there's a new variant being implemented in Jetpack to support Getty Images provider (more info in this PR), so it would be great to include it too.