Add .model.version, improve .urn

[khaeru]

These support easier and more regular handling of version and URN identifiers for SDMX objects.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.92%. Comparing base (473f3af) to head (4b129df).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #189      +/-   ##
==========================================
- Coverage   98.91%   97.92%   -1.00%     
==========================================
  Files          95       97       +2     
  Lines        7539     7699     +160     
==========================================
+ Hits         7457     7539      +82     
- Misses         82      160      +78     

Files	Coverage Δ
sdmx/model/common.py	99.90% <100.00%> (ø)
sdmx/model/version.py	100.00% <100.00%> (ø)
sdmx/tests/model/test_version.py	100.00% <100.00%> (ø)
sdmx/tests/test_docs.py	100.00% <100.00%> (ø)
sdmx/tests/test_urn.py	100.00% <100.00%> (ø)
sdmx/urn.py	100.00% <100.00%> (ø)

... and 18 files with indirect coverage changes