[Optimization] Use O(logn) complexity to get the Google Product Category String

Why here uses String value -> id format?
https://github.com/google/shoptimizer/blob/a9fff3ac203e39480dff7585fa37b999fbe7470e/shoptimizer_api/util/gpc_id_to_string_converter.py#L55-L65

If I have a `Google Product Categoty` = `6797`, Python needs to iterate `5437` values to find the String `食品・飲料 > 飲料 > バターミルク` , this will be a huge performance issue.
https://github.com/google/shoptimizer/blob/a9fff3ac203e39480dff7585fa37b999fbe7470e/shoptimizer_api/config/gpc_string_to_id_mapping_ja.json#L5438

So, from my opinion, we need to init an id -> String value dictionary, then use o(logn) complexity to get the String