Progressively Enhanced Server Actions (#26774)

Summary:
This automatically exposes `$$FORM_ACTIONS` on Server References coming
from Flight. So that when they're used in a form action, we can encode
the ID for the server reference as a hidden field or as part of the name
of a button.

If the Server Action is a bound function it can have complex data
associated with it. In this case this additional data is encoded as
additional form fields.

To process a POST on the server there's now a `decodeAction` helper that
can take one of these progressive posts from FormData and give you a
function that is prebound with the correct closure and FormData so that
you can just invoke it.

I updated the fixture which now has a "Server State" that gets
automatically refreshed. This also lets us visualize form fields.
There's no "Action State" here for showing error messages that are not
thrown, that's still up to user space.

DiffTrain build for commit facebook/react@aef7ce5.

Changelog: [Internal]
<< DO NOT EDIT BELOW THIS LINE >>

Reviewed By: christophpurrer

Differential Revision: D45548613

Pulled By: tyao1

fbshipit-source-id: 0d4206c18c4818fa410e8b18f3a0f0942237c91b

Author: sebmarkbage

packages/react-native/Libraries/Renderer/REVISION

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