ENH: Added get_database_metadata

[snowman2]

• Part of ENH: Expose PROJ database metadata #990
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

Merging #991 (6ba6617) into master (61ece93) will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #991      +/-   ##
==========================================
+ Coverage   96.14%   96.26%   +0.11%     
==========================================
  Files          20       20              
  Lines        1765     1765              
==========================================
+ Hits         1697     1699       +2     
+ Misses         68       66       -2     

Impacted Files	Coverage Δ
pyproj/crs/coordinate_operation.py	100.00% <0.00%> (+1.73%)	⬆️

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