Feature/externalize files from chatflow - do not save as base64

[vinodkiran]

Currently Flowise saves all uploaded docs as base64 string to the chatflow. It is not scalable with large documents. Issue - https://github.com/FlowiseAI/Flowise/issues/1247

Solution: save the files to a local path and only save the filepath to the chatflow.

This PR extracts the base64 string and saves it to the path specified in env.BLOB_STORAGE_PATH (creates a sub-folder with the chatflowid)

This is backwards compatible, when a current chatflow (with base64 in flowData) is opened and saved, the files are extracted and saved.