Use block support flag for hydration opt-in

[yscik]

Change the hard-coded list of supported blocks in the render_block hook.
Use a support flag, view, to enable frontend block hydration.

Also use an extend form of the flag to control the hydration technique setting.

At this point this is just a quick step to allow adding new blocks, I imagine the final API might look different.

[luisherranz]

Yeah, we need to start thinking about the opt-in mechanism 🙂

I don't like view that much, as that name is already used by people using viewScript to enqueue regular JS files and I think it can be confusing, but we can figure that out later.

Feel free to merge 👍