Improved type casting in the appender

[taniabogatsch]

Depends on #195.

Relevant changes

• Introduce a recursive tryCast function attempting to cast the driver.Value input of each column into the expected duckdb type.
• Support unmarshalled struct fields like map[string]any to simplify appending JSON inputs. Respective test: TestAppenderWithJSON.
• Throw a dedicated error message for mismatching struct fields.
• Moved nil checks into the respective setter functions.
• Return dedicated driverErr to indicate a bug in the driver (code should never reach this).