ENH: add graph validation function

[gboeing]

This PR adds a private internal _validate module and exposes public functions via the convert module. This allows users to validate their graphs and GeoDataFrames and, in turn, will make it easier for people manually create their own from raw data while ensuring it has all the required attributes that OSMnx expects (and adds automatically in the graph_from_x functions).

This ensures that all the "expected standard structure" exists in a graph if you've created it from raw data, hopefully making it much easier for users to do so without guessing at a checklist themselves.

• add private internal _validate module with validation functionality
• expose public validate_graph, validate_node_edge_gdfs, and validate_features_gdf functions via the convert module

See also #1314 and #1234.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (f98bae6) to head (5fb2de1).
⚠️ Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1317      +/-   ##
==========================================
+ Coverage   98.47%   98.57%   +0.09%     
==========================================
  Files          24       25       +1     
  Lines        2425     2591     +166     
==========================================
+ Hits         2388     2554     +166     
  Misses         37       37              

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