Support TfLite schema buffer and custom options offsets

[ddavis-2015]

@tensorflow/micro

Allow for models >2Gb (and less than 4Gb) in size, as generated by the TfLite converter. Parse TfLite schema Buffer tables where the offset and size fields are active. Parse TfLite schema Operator tables where the large_custom_options_offset and large_custom_options_size fields are active. Correctly process the Offline Memory Planner metadata buffer. Correctly process the compression metadata buffer. Add unit tests for all of the above.

bug=fixes #3196