[POC] Fix SonataAdminBundle 4.x integration

[wbloszyk]

Subject

POC for Sonata Admin 4.x integration.

The concept is based on the API upgrade from GatewayConfigAdmin to Sonata Admin 4.x and changing the config form field from GatewayConfigType to a dedicated FormType from config.

Example:

Closes #478.

Changelog

### Added
- Added `payum.gateway_config_type` 
### Removed
- Removed `Payum\Bundle\PayumBundle\Form\Type\GatewayConfigType`
### Fixed
- Sonata Admin integration

To do

• Add GatewayConfigType as payum.gateway_config_type default value
• Update the tests;
• Update the documentation;
• Add an upgrade note.