Embed the install manifests in flux binary

- add make target for generating the install manifests using kustomize
- embed the generated manifests in flux binary
- the install and bootstrap commands default to using the embedded manifests
- download the install manifests from GitHub only if the install/bootstrap version arg is set

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/version.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/fluxcd/flux2/internal/utils"
+	"github.com/fluxcd/flux2/pkg/manifestgen/install"
+)
+
+func getVersion(input string) (string, error) {
+	if input == "" {
+		return rootArgs.defaults.Version, nil
+	}
+
+	if isEmbeddedVersion(input) {
+		return input, nil
+	}
+
+	var err error
+	if input == install.MakeDefaultOptions().Version {
+		input, err = install.GetLatestVersion()
+		if err != nil {
+			return "", err
+		}
+	} else {
+		if ok, err := install.ExistingVersion(input); err != nil || !ok {
+			if err == nil {
+				err = fmt.Errorf("targeted version '%s' does not exist", input)
+			}
+			return "", err
+		}
+	}
+
+	if !utils.CompatibleVersion(VERSION, input) {
+		return "", fmt.Errorf("targeted version '%s' is not compatible with your current version of flux (%s)", input, VERSION)
+	}
+	return input, nil
+}
+
+func isEmbeddedVersion(input string) bool {
+	return input == rootArgs.defaults.Version
+}