[c++] Methodize timestamped-schema-evolution factory

[johnkerl]

Issue and/or context: This is a line-count-reducing under-diff split out from #2785 for issue #2407; [sc-51048].

Changes:

On #2895 I made an inline code block for instantiating an ArraySchemaEvolution object pointed at the current array's context and open-at timestamp (if any). That was after I did a call-graph analysis that showed that this was the sole spot in main -- at that point in time -- where we needed to do this.

However, on #2785 we'll be making more use of schema evolution, from more than one callsite. It makes even more sense now to encapsulate this logic in a single place.

Notes for Reviewer:

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 59.52%. Comparing base (bdba3c3) to head (4a16241).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2909       +/-   ##
===========================================
- Coverage   89.88%   59.52%   -30.37%     
===========================================
  Files          37       92       +55     
  Lines        3925    11337     +7412     
  Branches        0      724      +724     
===========================================
+ Hits         3528     6748     +3220     
- Misses        397     4418     +4021     
- Partials        0      171      +171     

Flag	Coverage Δ
libtiledbsoma	43.36% <50.00%> (?)
python	90.03% <ø> (+0.15%)	⬆️

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Components	Coverage Δ
python_api	90.03% <ø> (+0.15%)	⬆️
libtiledbsoma	50.00% <50.00%> (∅)