Allow support powers to require tech prerequisites.

Rewritten version of #5374.  This allows us to specify multiple prerequisites for a support power, and is the first step towards moving the shared powers onto the player actor (simplifying the code and making it consistent with production).

Two simple testcases:

* Add `Prerequisites: dome` to `MSLO -> NukePower` in RA for a nuke that also requires radar to charge.
* Move `HQ -> AirstrikePower` to `player` in TD for shared powers.