(MINOR) Refactor how property values are returned from accessor functions

[scouten-adobe]

Changes in this pull request

Introduces XmpValue as a templated type with accessors for the various flags that the C++ XMP Toolkit can return.

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 #81 (775ef6c) into main (4f3f10e) will increase coverage by 1.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   89.96%   91.74%   +1.77%     
==========================================
  Files           6        7       +1     
  Lines         279      339      +60     
==========================================
+ Hits          251      311      +60     
  Misses         28       28              

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

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