feat: add latest image selector; reimplement label selection

This MR improves image selection by:

• Supporting the full spectrum of label-selectors (see https://docs.hetzner.cloud/#label-selector)
• Supporting the :latest suffix which selects the newest image matching the content before suffix.

Additionally, basic integration tests as well as unit tests for image selection are added.

Signed-off-by: Florian Bauer florian@fsrv.xyz