Add prepared_schema() to get Arrow schema without executing statement

[fvaleye]

Summary

Adds a new prepared_schema() method that extracts the Arrow Schema from a prepared statement without executing it.

When using stream_arrow() for streaming query results, the caller must provide the schema upfront.
Currently, the only way to obtain the schema is to execute the query first, which forces a pattern like:

// Current workaround: prepare and execute twice 
let mut stmt1 = conn.prepare(sql)?;
stmt1.execute([])?; 
let schema = stmt1.schema();
drop(stmt1); 

let mut stmt2 = conn.prepare(sql)?;  // Second preparation        
let stream = stmt2.stream_arrow([], schema)?;

This doubles the preparation cost (parsing, planning, optimization) for every streaming query.
With prepared_schema(), the schema can be obtained directly from the prepared statement:

// New: single preparation                                                                                                                                                            
let mut stmt = conn.prepare(sql)?;                                                                                                                                                    
let schema = stmt.prepared_schema()?.unwrap();                                                                                                                                        
let stream = stmt.stream_arrow([], schema)?;