Add metadata structures and sets to IM and SDMX-ML reader

[khaeru]

This adds coverage of some IM classes previously missing, particularly from §7: meta data structures and metadata sets; and reading these from SDMX-ML 2.1 and 3.0.

• Closes Implement MetadataSet, MetadataStructureDefinition, etc. #73.
• Partly addresses:
  • Expand Information Model coverage #85
  • Implement/ensure compat with SDMX 3.0 #87

PR checklist

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

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (83ec5db) 98.36% compared to head (577c7f7) 96.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
- Coverage   98.36%   96.35%   -2.01%     
==========================================
  Files          87       87              
  Lines        6480     6916     +436     
==========================================
+ Hits         6374     6664     +290     
- Misses        106      252     +146     

Files	Coverage Δ
sdmx/format/xml/common.py	100.00% <100.00%> (ø)
sdmx/format/xml/v21.py	100.00% <ø> (ø)
sdmx/format/xml/v30.py	100.00% <ø> (ø)
sdmx/message.py	100.00% <100.00%> (ø)
sdmx/model/common.py	99.90% <100.00%> (+<0.01%)	⬆️
sdmx/model/v21.py	100.00% <100.00%> (ø)
sdmx/model/v30.py	100.00% <100.00%> (ø)
sdmx/reader/xml/v30.py	95.23% <100.00%> (+0.28%)	⬆️
sdmx/rest.py	100.00% <ø> (ø)
sdmx/source/__init__.py	100.00% <ø> (ø)
... and 9 more

... and 16 files with indirect coverage changes