Add support for specifying a version constraint when updating plugins,
matching the existing behavior of helm plugin install.
Changes:
- Add --version flag to plugin update command
- Update VCSInstaller.Update() to resolve and checkout specified version
- Update FindSource() to accept version parameter
- Add TestVCSInstallerUpdateWithVersion test for version support
Signed-off-by: MrJack <36191829+biagiopietro@users.noreply.github.com>