cli/command/completion: add Platforms

Add a utility for completing platform strings.

Platforms offers completion for platform-strings. It provides a non-exhaustive
list of platforms to be used for completion. Platform-strings are based on
[runtime.GOOS] and [runtime.GOARCH], but with (optional) variants added. A
list of recognised os/arch combinations from the Go runtime can be obtained
through "go tool dist list".

Some noteworthy exclusions from this list:

  - arm64 images ("windows/arm64", "windows/arm64/v8") do not yet exist for windows.
  - we don't (yet) include `os-variant` for completion (as can be used for Windows images)
  - we don't (yet) include platforms for which we don't build binaries, such as
    BSD platforms (freebsd, netbsd, openbsd), android, macOS (darwin).
  - we currently exclude architectures that may have unofficial builds,
    but don't have wide adoption (and no support), such as loong64, mipsXXX,
    ppc64 (non-le) to prevent confusion.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/completion/functions_test.go

@@ -0,0 +1,15 @@
+package completion
+
+import (
+	"testing"
+
+	"github.com/spf13/cobra"
+	"gotest.tools/v3/assert"
+	is "gotest.tools/v3/assert/cmp"
+)
+
+func TestCompletePlatforms(t *testing.T) {
+	values, directives := Platforms(nil, nil, "")
+	assert.Check(t, is.Equal(directives&cobra.ShellCompDirectiveNoFileComp, cobra.ShellCompDirectiveNoFileComp), "Should not perform file completion")
+	assert.Check(t, is.DeepEqual(values, commonPlatforms))
+}