Add support for consuming images from community galleries

[invidian]

/kind feature

Describe the solution you'd like

Recently made available in public preview community galleries feature for consuming the SIG images from SIGs in other tenants and subscriptions seem very useful for 3rd parties to publish their own images for public consumption, so there is no need to publish in the marketplace, which as far as I know is a complex process and also specifically to add the CI for Flatcar PR/templates #1729.

Anything else you would like to add:

It seems this requires use of dedicated API field in ImageReference to consume those images. We may want to ensure we use new enough SDK version and expose those fields in CAPZ CRDs to make it available for consumption.

CC @t-lo @jepio

[invidian]

/assign

[invidian]

For the time being and for e2e coverage, I pushed the code to #1729, though e2e are still failing, I'm investigating.