Add rails db:system:change command

Add `rails db:system:change` command for changing databases.

```
bin/rails db:system:change --to=postgresql
   force  config/database.yml
    gsub  Gemfile
```

The change command copies a template `config/database.yml` with
the target database adapter into your app, and replaces your database
gem with the target database gem.

1 files changed, 20 insertions, 0 deletions

diff --git a/railties/lib/rails/commands/db/system/change/change_command.rb b/railties/lib/rails/commands/db/system/change/change_command.rb
new file mode 100644
index 0000000000..760c229c07
--- /dev/null
+++ b/railties/lib/rails/commands/db/system/change/change_command.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require "rails/generators"
+require "rails/generators/rails/db/system/change/change_generator"
+
+module Rails
+  module Command
+    module Db
+      module System
+        class ChangeCommand < Base # :nodoc:
+          class_option :to, desc: "The database system to switch to."
+
+          def perform
+            Rails::Generators::Db::System::ChangeGenerator.start
+          end
+        end
+      end
+    end
+  end
+end