Parse FIB AutoTEM metadata (#786)

* Renamed 'self._milling' to 'self._milling_images' and 'MillingProgress' to 'MillingImage'
* Added new helper functions, classes, and lookup tables needed to parse and extract FIB AutoTEM metadata
* Added placeholder endpoint for use with registering milling progress
* Added logic to FIBContext to post milling progress to backend
* Added tests

Author: tieneupin