* Add tests, examples and documentation changes for custom ONNX operators registration mechanism
* Change snippet paths
* fix CoreThreadingTests.ReadNetwork - data race in ops_bridge
* Make TemplateExtension::Operation externally visible
* changes after review
* apply code format
* use std::int64_t
* forward declare get_attribute_value specializations
* introduce unregister_operator in onnx_importer
* onnx_custom_op - lock mem first then take a buffer
* func tests - create template_extension via make_so_pointer
* fix build with NGRAPH_ONNX_IMPORT_ENABLE=OFF
* remove exports from Operation and Extension
* Move multithreaded AddExtension test to different directory to it can be excluded when NGRAPH_ONNX_IMPORT_ENABLE=OFF
* Dont include Extension tests if ENABLE_MKL_DNN=OFF
* fix excluding onnx_reader tests
* include extension tests only if mkl is enabled
* add comment on empty blob
* use register_operator conditionally in template_extension
* fix docs after review
* create static library from onnx_custom_op
* add additional test for unregister_operator
* move model example after register step
* revert changes to unit tests
* update ngraphConfig.cmake.in header
* add headers to onnx_custom_op
* changes to docs CMakeLists
* remove redundant onnx_importer dependency
* remove extension directory from func tests
* make onnx_importer a component of ngraph package
* docs fixes
* update header of ngraph/cmake/share/ngraphConfig.cmake.in with ngraph_onnx_importer_FOUND
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