feat(billing): Add cli subcommand for monthly active user counts

internal/tests/cli/boundary/_billing.bash

@@ -0,0 +1,18 @@
+# Copyright (c) HashiCorp, Inc.
+# SPDX-License-Identifier: BUSL-1.1
+
+function active_users_last_two_months() {
+  boundary billing monthly-active-users -format json
+}
+
+function active_users_start_time() {
+  boundary billing monthly-active-users -start-time=$1 -format json
+}
+
+function active_users_start_time_and_end_time() {
+  boundary billing monthly-active-users -start-time=$1 -end-time=$2 -format json
+}
+
+function active_users_end_time() {
+  boundary billing monthly-active-users -end-time=$1 -format json
+}