Use ConnectionStrings section with named connection strings

[satano]

We use our own setup of connection string in settings. We require ConnectionString section with ProviderName and ConnectionString subkeys. (And I think when we use migrations in APIs, the connection string is twice in the appsettings.json.)

We could use default connection strings settings (as Entity Framework does it). The section name is ConnectionStrings and subkeys are names of the connection strings. Our provider can be integrated in connection string itself. When creating IDatabase, connection string name will be specified. If the name is not specified, default name DefaultConnection will be used.