Make the main branches tunable

Some consumers of modulesync want to enable this for e.g. a `develop`
branch, so make it tunable and use the `main` and `master` names as
default value for this new setting which should work out-of the box for
most cases.

config_defaults.yml

@@ -5,6 +5,7 @@
   pidfile_workaround: false
   additional_packages: ''
   acceptance_tests: true
+  main_branches: ['main', 'master']
 # PDK creates this
 .github/workflows/puppet-lint.yml:
   delete: true

moduleroot/.github/workflows/ci.yml.erb

@@ -8,8 +8,9 @@ on:
   pull_request: {}
   push:
     branches:
-      - main
-      - master
+<%- @configs['main_branches'].each do |main_branch| -%>
+      - <%= main_branch %>
+<%- end -%>
 
 concurrency:
   group: ${{ github.ref_name }}