Create an allowed_default_active= (or similar) list to prevent auto-active backends #31

Right now, my choice was to trust backends, if they say:

requires_opt_in = False

then the backend will be active and may be chosen without any further user input (environment variable, backend_opts).

But at EuroSciPy there were some quite strong opinions that we shouldn't trust backends that much, I think one solution for that is to add an option:

allowed_default_active=["name1", "name2"]

to the BackendSystem creation. If a backend name doesn't show up, just ignoring the requires_opt_in=False request should work OK.

Not passing it would just keep allowing it, so that it really becomes the libraries choice.

N.B.: Related, I suspect we'll want a blocklist as well. The reason isn't even fear of bad actors, but protecting users of bugs (which are always possible).
But that is a larger topic (one could even have a fine-grained block-list, etc.), and I don't think it is a "now" problem.