Ability to tag host machines

As a user I want to be able to tag my host machines based on usage add host label selector when defining a new cluster.

Example:
I want a cluster, but microvms should be created only on hosts where it has "testenv" label.

Nice to have:
Available microvm providers are automatically tagged. When flintlock starts it can check what binaries are available.

No files, tests, or concrete entry points are named. First clarify the host-label and provider auto-discovery design, then trace the existing cluster creation and host selection paths. Done means a cluster can select hosts by labels, with automatic provider tags treated as an optional enhancement.

Written by the indexing model from the issue text.