feat(ffi)!: do not panic in visit_scan_metadata

[aleksandarskrbic]

What changes are proposed in this pull request?

Fixes #680

Avoid .unwrap() in visit_scan_metadata, replace return parameter to ExternResult<NullableCvoid>

Usage example in C:

  ExternResultNullableCvoid result = visit_scan_metadata(scan_metadata, context->engine, engine_context, scan_row_callback);
  if (result.tag != OkNullableCvoid) {
    printf("Error visiting scan metadata\n");
    exit(-1);
  }

How was this change tested?

Running read_table.c example

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.75%. Comparing base (3bcd274) to head (8dff08a).

Files with missing lines	Patch %	Lines
ffi/src/scan.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1526      +/-   ##
==========================================
- Coverage   84.76%   84.75%   -0.01%     
==========================================
  Files         122      122              
  Lines       33032    33035       +3     
  Branches    33032    33035       +3     
==========================================
  Hits        28000    28000              
- Misses       3659     3662       +3     
  Partials     1373     1373              

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