main: add support for CLI extensions via external binaries

This adds some logic to detect and dispatch unknown subcommands to
extensions available in `$PATH`. Additional commands can be
implemented by adding relevant `ostree-$verb` binaries to the system.

As an example, if a `/usr/bin/ostree-extcommand` extension is provided,
the execution of `ostree extcommand --help` will be dispatched to that
as `ostree-extcommand extcommand --help`.

tests/test-cli-extensions.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+#
+# Copyright (C) 2021 Red Hat Inc.
+# SPDX-License-Identifier: LGPL-2.0+
+
+set -euo pipefail
+
+. $(dirname $0)/libtest.sh
+
+echo '1..2'
+
+mkdir -p ./localbin
+export PATH="./localbin/:${PATH}"
+ln -s /usr/bin/env ./localbin/ostree-env
+${CMD_PREFIX} ostree env --help >out.txt
+assert_file_has_content out.txt "with an empty environment"
+rm -rf -- localbin
+
+echo 'ok CLI extension localbin ostree-env'
+
+if ${CMD_PREFIX} ostree nosuchcommand 2>err.txt; then
+    assert_not_reached "missing CLI extension ostree-nosuchcommand succeeded"
+fi
+assert_file_has_content err.txt "Unknown command 'nosuchcommand'"
+rm -f -- err.txt
+
+echo 'ok CLI extension unknown ostree-nosuchcommand'