refactor!: Rename template to generate and move rebase/upgrade under switch (#116)

This updates the `template` subcommand to be `generate`. The `template`
usage will continue to work as an alias to `generate`. A new `switch`
command is added that will manage both `rpm-ostree rebase` and
`rpm-ostree upgrade` and is fully replacing the respective subcommands
as a breaking change.

The new `switch` command is under the feature flag `switch` and will
currently only build for the `main` branch builds until it is moved as a
default feature (`v0.9.0`).

Closes #159

integration-tests/mock-scripts/buildah

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 print_version_json() {
   local version="1.24.0"
@@ -8,6 +8,11 @@ print_version_json() {
 main() {
   if [[ "$1" == "version" && "$2" == "--json" ]]; then
     print_version_json
+  elif [[ "$1" == "build" && "$6" == *"cli_test.tar.gz" ]]; then
+    tarpath=$(echo "$6" | awk -F ':' '{print $2}')
+    echo "Exporting image to a tarball (JK JUST A MOCK!)"
+    echo "${tarpath}"
+    touch $tarpath
   else
     echo 'Running buildah'
   fi