pkg/imagefilter/formatter: add shell output

[schuellerf]

This should implement the initial idea of a
copy'n'paste ready list of possibilities.

[mvo5]

This is very nice, thank you!

I would love a test as well, maybe something like

index c866d2a30..40640261a 100644
--- a/pkg/imagefilter/formatter_test.go
+++ b/pkg/imagefilter/formatter_test.go
@@ -65,6 +65,11 @@ func TestResultsFormatter(t *testing.T) {
                        },
                        `[{"distro":{"name":"test-distro-1"},"arch":{"name":"test_arch3"},"image_type":{"name":"qcow2"}},{"distro":{"name":"test-distro-1"},"arch":{"name":"test_arch"},"image_type":{"name":"test_type"}}]` + "\n",
                },
+               {
+                       "shell",
+                       []string{"test-distro-1:qcow2:test_arch3"},
+                       "test-distro-1 --type type:qcow2 --arch arch:test_arch3\n",
+               },
        } {
                res := make([]imagefilter.Result, len(tc.fakeResults))
                for i, resultSpec := range tc.fakeResults {

(I did not test this) is already enough?

[schuellerf]

of course I missed the test 😭 sorry

[supakeen]

I believe you can do return errors.Join(errs...) directly here. errors.Join discards nil-values in errors and returns nil when it only has nils in its arguments (or when called with an empty slice).

[mvo5]

Thank you for adding the test. This looks fine to me now.