Add static model provider for devices without get-schema support

[ghost]

• fixes List with identityref key is not being encoded correctly for python bundle #718
  Also
• fixes Enum classes should be clearly documented #716
• fixes CRUD service unrecognized #554

[codecov]

Codecov Report

Merging #721 into master will increase coverage by 0.52%.
The diff coverage is 86.3%.

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage    79.6%   80.13%   +0.52%     
==========================================
  Files         173      173              
  Lines       16395    16424      +29     
  Branches     1471     1470       -1     
==========================================
+ Hits        13051    13161     +110     
+ Misses       2987     2906      -81     
  Partials      357      357

Impacted Files	Coverage Δ
ydkgen/printer/python/enum_printer.py	97.36% <ø> (+17.77%)	⬆️
ydkgen/printer/meta_data_util.py	82.51% <100%> (ø)	⬆️
...hon/core/tests/test_sanity_type_mismatch_errors.py	87.77% <100%> (ø)	⬆️
sdk/cpp/core/src/path/netconf_model_provider.cpp	94.91% <100%> (+1.03%)	⬆️
sdk/cpp/core/tests/core_test.cpp	81.25% <60%> (-18.75%)	⬇️
sdk/cpp/core/src/path/netconf_session.cpp	86.23% <80%> (-0.54%)	⬇️
sdk/python/core/tests/test_sanity_levels.py	98.29% <96%> (+15.51%)	⬆️

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