Add APIs to allow iteration over the XMP data model

[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 #141 (ba307f4) into main (8a47eb8) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   97.54%   97.72%   +0.18%     
==========================================
  Files           7        8       +1     
  Lines        1302     1406     +104     
==========================================
+ Hits         1270     1374     +104     
  Misses         32       32              

Impacted Files	Coverage Δ
src/ffi.rs	100.00% <ø> (ø)
src/lib.rs	100.00% <ø> (ø)
src/xmp_iterator.rs	100.00% <100.00%> (ø)
src/xmp_meta.rs	99.78% <100.00%> (+<0.01%)	⬆️
src/xmp_value.rs	100.00% <100.00%> (ø)

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