[firebase_data_connect] Response data Correct UTF-8 decoding for international characters

[ska2519]

Flutter package: firebase_data_connect: ^0.1.4
Development environment: Firebase emulator

When querying files in the dataconnect-generated folder generated by firebase_data_connect, English is printed normally, but other characters such as Korean are broken.

It seems that charset=utf-8 is not set in the server's response type.

Originally posted by @ska2519 in #17281