Add XmpMeta::contains_struct_field

[scouten-adobe]

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[codecov]

Codecov Report

Merging #106 (45d64c5) into main (c0531e6) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   96.02%   96.14%   +0.12%     
==========================================
  Files           7        7              
  Lines         755      779      +24     
==========================================
+ Hits          725      749      +24     
  Misses         30       30              

Impacted Files	Coverage Δ
src/ffi.rs	100.00% <ø> (ø)
src/xmp_meta.rs	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more