Tests and docs for registering custom ONNX operators (#2416)

Tests and docs for registering custom ONNX operators (#2416)

* 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