This adds a v1alpha2 API, without changing the controller or tests to
use it (yet). The new API has roughly the desired shape, per
https://github.com/fluxcd/flux2/discussions/1124, but supporting only
things that the controller supports now.
It's necessary to give the v1alpha1 type a `storageversion` marker so
that 1. code generation keeps working, and 2. tests still work, since
they still expect v1alphav1 types. v1alpha1 will be removed once the
controller and tests are ported to v1alpha2.
Signed-off-by: Michael Bridgen <michael@weave.works>