add extensions for registration callback for Animatable2 and LayoutTransition

[alterevit]

No description provided.

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[alterevit]

I signed it!

[googlebot]

CLAs look good, thanks!

[jaredsburrows]

Can you provide tests?

[alterevit]

Yes, a bit later

[alterevit]

@jaredsburrows
do I need to do something else for approve?

[jaredsburrows]

@alterevit I do not own this repository.