Add XmpMeta::append_array_item

[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 #117 (fbb68f1) into main (01ef0c4) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
+ Coverage   96.32%   96.40%   +0.08%     
==========================================
  Files           7        7              
  Lines         870      891      +21     
==========================================
+ Hits          838      859      +21     
  Misses         32       32              

Impacted Files	Coverage Δ
src/ffi.rs	100.00% <ø> (ø)
src/xmp_meta.rs	99.62% <100.00%> (+0.01%)	⬆️

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