Rename `TfLiteRegistrationExternal` as `TfLiteOperator`.
Rename `TfLiteRegistrationExternal` as `TfLiteOperator`.
Likewise rename corresponding API functions (all experimental).
Rename (experimental) `TfLiteInterpreterOptionsAddRegistrationExternal` as `TfLiteInterpreterOptionsAddOperator`.
Rename (private) header registration_external.h as operator.h and
rename registration_external.cc as operator.cc.

Also add temporary backwards compatibility aliases that forward the old
names to the new names. These backwards compatibility aliases are needed
even though these names are experimental, because the old names are currently
referenced in MediaPipe, which is in a separate repository, and thus can't be
updated atomically.
