show active balder plugins in console output

[matosys]

Is your feature request related to a problem? Please describe.

If you start working with plugins, sometimes it sometimes becomes unclear which one are active.

Describe the solution you'd like

For this I would suggest to add a new line in the Balder script output header, which looks like the following:

+----------------------------------------------------------------------------------------------------------------------+
| BALDER Testsystem                                                                                                    |
|  python version 3.10.6 (main, Nov 14 2022, 16:10:14) [GCC 11.3.0] | balder version 0.0.1b2                           |
|                                                                                                                      |
|  plugins: `balderplugin.junit.JUnitPlugin`, `balderglob.MyCustomPlugin`                                              |
+----------------------------------------------------------------------------------------------------------------------+
Collect 1 Setups and 5 Scenarios
  resolve them to 5 mapping candidates

================================================== START TESTSESSION ===================================================

The active plugins are available over the PluginManager (session manager instance available over balder_session.plugin_manager).