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/testdata/envsubst/file.gold

@@ -0,0 +1,10 @@
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: test
+  namespace: flux-system
+spec:
+  ref:
+    branch: main
+  interval: 5m
+  url: ssh://git@github.com/example/test

cmd/flux/testdata/envsubst/file.yaml

@@ -0,0 +1,10 @@
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: ${REPO_NAME}
+  namespace: ${REPO_NAMESPACE:=flux-system}
+spec:
+  ref:
+    branch: main
+  interval: 5m
+  url: ssh://git@github.com/example/${REPO_NAME}