This PR adds a couple methods to the C-API to create and consume a stream query result.

The pending family of functions has a new addition:

duckdb_state duckdb_pending_prepared_streaming(duckdb_prepared_statement prepared_statement,
                                               duckdb_pending_result *out_result);

This creates a duckdb_pending_result, like before, the pending query result can be partially executed with
duckdb_pending_execute_task.
The only difference is that to execute the pending query and turn it into a streaming query result, the following function needs to be called.

duckdb_state duckdb_create_streaming_result(duckdb_pending_result pending_result, duckdb_result *out_result);

Similar to duckdb_execute_pending this creates a result object out of the pending query result.
(also note that this pending query result can not be turned into a regular result)

To interact with the result, most of the materialized-result family of functions can be used, with the exception of the chunk/row related functions.
To get a chunk out of the stream query result, instead you have to use:

duckdb_data_chunk duckdb_stream_fetch_chunk(duckdb_result result);