Recover IptDataSets::recordId in the API

[piponazo]

As it has been pointed out by @jim-easterbrook here , IptcDataSets::recordId was declared in datasets.hpp but not defined in datasets.cpp (it was actually defined but just as an implementation detail).

In #2060 I was too aggressive and not careful with some changes in the API. That's why I am recovering that function as part of the API.

[codecov]

Codecov Report

Merging #2137 (c1ae1bc) into main (89e66ff) will not change coverage.
The diff coverage is 72.72%.

@@           Coverage Diff           @@
##             main    #2137   +/-   ##
=======================================
  Coverage   63.35%   63.35%           
=======================================
  Files          97       97           
  Lines       19174    19174           
  Branches     9712     9712           
=======================================
  Hits        12148    12148           
  Misses       4758     4758           
  Partials     2268     2268           

Impacted Files	Coverage Δ
src/datasets.cpp	93.03% <72.72%> (ø)

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