Re-implement Server Provider architecture
Today's implementation has a mesh of GObject and C struct with virtual methods, that makes hard and easy to fail to implement providers.
Road 1: GdaServerProvider should be converted to a GInterface
Add to GdaServerProvider, all abstract methods to implement a provider, making public the ones actually are and private the ones to be implemented by providers.
Road 2: GdaServerProvider should be converted to a GInterface
A new GdaProviderBase abstract class is needed, so connections, parsers and operations can work on it. This class should implement GdaServerProvider interface.
Each provider should implement GdaProviderBase.