Add flux envsubst command

This command can be used to replicate the behavior of the
Flux Kustomization post-build substitutions.

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

cmd/flux/envsubst_test.go

@@ -0,0 +1,50 @@
+/*
+Copyright 2024 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package main
+
+import (
+	"bytes"
+	"os"
+	"testing"
+
+	. "github.com/onsi/gomega"
+)
+
+func TestEnvsubst(t *testing.T) {
+	g := NewWithT(t)
+	input, err := os.ReadFile("testdata/envsubst/file.yaml")
+	g.Expect(err).NotTo(HaveOccurred())
+
+	t.Setenv("REPO_NAME", "test")
+
+	output, err := executeCommandWithIn("envsubst", bytes.NewReader(input))
+	g.Expect(err).NotTo(HaveOccurred())
+
+	expected, err := os.ReadFile("testdata/envsubst/file.gold")
+	g.Expect(err).NotTo(HaveOccurred())
+	g.Expect(output).To(Equal(string(expected)))
+}
+
+func TestEnvsubst_Strinct(t *testing.T) {
+	g := NewWithT(t)
+	input, err := os.ReadFile("testdata/envsubst/file.yaml")
+	g.Expect(err).NotTo(HaveOccurred())
+
+	_, err = executeCommandWithIn("envsubst --strict", bytes.NewReader(input))
+	g.Expect(err).To(HaveOccurred())
+	g.Expect(err.Error()).To(ContainSubstring("variable not set (strict mode)"))
+}