Add ability to query current value of parallel workers for an attachment

[hvlad]

No description provided.

[hvlad]

See also discussion in fb-devel from 2 Apr 2023, thread "Query current value of parallel workers for an attachment"

[hvlad]

1. New column MON$PARALLEL_WORKERS INTEGER were added into MON$ATTACHMENTS table.
2. New variable PARALLEL_WORKERS is available in SYSTEM context.
3. New tag fb_info_parallel_workers (value 149) is available in IAttachment::getInfo() and isc_database_info() calls.

[pavel-zotov]

@@@ QA issue @@@

Currently test checks only ability to query values from mon$attachments and rdb$get_context.
It does NOT check tag fb_info_parallel_workers because this is not yet implemented
(BTW, 'isc_dpb_parallel_workers' also not yet avaliable in the firebird-driver).