chore: import upstream snapshot with attribution

tensorflow/lite/delegates/external/README.md

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+# What is an External Delegate?
+An external delegate is a special Tensorflow Lite delegate that is simply
+initialized from loading a dynamic library which encapsulates an actual
+Tensorflow Lite delegate implementation. The actual delegate exposes the
+following two creation and deletion C APIs:
+
+* __tflite_plugin_create_delegate__ (declaration seen below) creates a delegate
+object based on provided key-value options. It may return NULL to indicate an
+error with the detailed information reported by calling `report_error` if
+provided. Each option key and value should be null-terminated.
+
+```
+TfLiteDelegate* tflite_plugin_create_delegate(
+  char** options_keys, char** options_values, size_t num_options,
+  void (*report_error)(const char *))
+```
+
+* __tflite_plugin_destroy_delegate__ (declaration seen below) destroys the
+delegate object that is created by the previous API. NULL as an argument value
+is allowed.
+
+```
+void tflite_plugin_destroy_delegate(TfLiteDelegate* delegate)
+```
+
+The external delegate provides an opaque and transparent way to utilize a
+Tensorflow Lite delegate when performing inference. In other words, one may
+replace the actual Tensorflow Lite delegate by simply updating the dynamic
+library without changing the application code. We developed this mainly for
+delegate evaluation.
+
+Note, this delegate is the corresponding C++ implementation to the one for
+Tensorflow Lite Python binding as shown [here](https://github.com/tensorflow/tensorflow/blob/7145fc0e49be01ef6943f4df386ce38567e37797/tensorflow/lite/python/interpreter.py#L42).