Fix(duckdb): Only SET connector_config values on cursor init if they are different

[erindru]

Prior to this, a duckdb connection config like:

type: duckdb
connector_config:
  temp_directory: /tmp/duckdb/
  memory_limit: '1GB'

would result in a SET call for each item in connector_config every time a new cursor was initialized.

This isnt a problem for many properties, but ones like temp_directory have side effects. If DuckDB has had to spill data to disk (and created files in temp_directory), if you try to SET temp_directory='...', even to the same value, it will throw an error:

duckdb.duckdb.NotImplementedException: Not implemented Error: Cannot switch temporary directory after the current one has been used

This PR changes the cursor init to read the existing values and only make SET calls if the configured values differ from what is already set

[themisvaltinos]

Suggested change

	existing_values = {r[0]: r[1] for r in cursor.fetchall()}
	existing_values = {field: setting for field, setting in cursor.fetchall()}

nit: maybe something more explicit like this so that it's more clear when going through the code

[erindru]

thanks, it didnt even occur to me to unpack the tuple like that. will update

[themisvaltinos]

Suggested change

	in_part = ",".join(["?" for _ in range(len(option_names))])
	in_part = ",".join("?" for _ in option_names)

nit: this should achieve the same