SQL: Add `probe` setting to `IMPORT DATABASE` statement

[gramian]

I propose to add a probe setting to the IMPORT DATABASE SQL statement, i.e.:

IMPORT DATABASE url WITH probe = true;

This probe setting is boolean and by default false. If set to true, the url argument is tested; in case of a file, if the file exists and is readable, in case of a link, if a 2XX status is returned (maybe using the HTTP method HEAD?).

This setting would allow testing an import source without potenitally modifying the database.

[lvca]

Closed by @gramian with #1401