Extend discovery Core interface to handle external discoveries

[lubomirw]

Add possibility to run discoveries manually in external provider. To achieve that, discovery Core interface and DTOs representing discovery needs to be extended:

• discovery can be created as external (bool)
• add endpoint to get discovery configuration - GET DiscoveryRequestDto
• add endpoint to import external discovery result - POST DiscoveryProviderDto
• add endpoint to import externally discovered certificates - POST List<DiscoveryProviderCertificateDataDto>