Adds debug logging for creation + completion

Changes

This adds the option to log to a local file called modelgateway_debug.log by setting an .env variable

LOG_TO_FILE=true

Adds multiple debug logging calls, so both the input to the model gateway is visible, the final call to Google Vertex and the response (text + score).

Example using https://github.com/brocode/fblog :

tail -f modelgateway_debug.log | fblog -a prefix -a suffix -a current_file_name -a suggestion -a language -a input -a parameters -a score -a exception

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@achueshev: Several changes made additionally - !359 (comment 1545461593)