Implements https://github.com/Azure/setup-kubectl/issues/259
- Add `version-file` input to action.yml that accepts a path to a
.tool-versions file (asdf/mise compatible format)
- Add `parseToolVersionsFile()` helper that reads kubectl version
from a .tool-versions file, skipping comments and blank lines
- Update `run()` to check `version-file` before `version`; when
provided, the resolved version goes through `resolveKubectlVersion()`
supporting both full (1.27.15) and major.minor (1.27) forms
- Add unit tests for `parseToolVersionsFile()` and the new run() path
