Feature/preflight extension check

[DmitriiAn]

1. Extension compatibility check : compares source vs target extensions, fails if target is missing any
2. filters.ini integration : reads [exclude-extension] from ~/filters.ini and skips those extensions from the check; also added a separate check that ~/filters.ini exists

[DmitriiAn]

@teknogeek0 pg_extension is a per-database catalog that only lists extensions installed. No change needed

[DmitriiAn]

I need to modify the logic to check for available extensions rather than installed ones, since pgcopydb can handle the installation

[DmitriiAn]

the logic adjusted: check available extensions on target (pg_available_extensions) instead of installed ones. Ready to be merged @teknogeek0