Add native support for generation 1 and 2 OS images and machine types

[vpnachev]

How to categorize this issue?

/area os
/kind enhancement
/platform azure

What would you like to be added:
Azure have two different generations of VM types - generation 1 and generation 2, see the docs for more details . Usually, each generation needs specific OS image and the two images are not compatible, e.g. gen1 image cannot boot gen2 VM and vice versa. It would be nice each OS name+version to refer to both images and hide this implementation detail from the shoot owner.

Why is this needed:
Currently this is done via separate versions using semver suffixes, e.g. 1.2.3 and 1.2.3-gen2. This causes some troubles like

• for given Major.Minor version it is not allowed to have more than one version with explicit supported classification.
• gardener maintenance controller might wrongly update 1.2.3-gen1 to 1.3.2-gen2 which results in broken clusters