Configure log level within the Cortex API spec config

[RobertLucian]

Description

Add a field in the cortex.yaml API spec that allows the user to set the log level for their API deployments:

- name: <string>  # API name (required)
  predictor:
    ...
    log_level: info # allow CRITICAL, ERROR, WARNING, INFO, DEBUG, NOTSET
    ... 

The default log level set would be INFO.

Consider adding a guide to showcase using cx_logger() in predictor implementations.

Motivation

To prevent spamming the log lines with unnecessary information. As pointed out by @vackosar.

Issues to sort out

1. Looks like uvicorn ignores our log levels from log_config.yaml, and for that, we'd have to have something like:
   logs with FastAPI and Uvicorn tiangolo/fastapi#1508 (comment)
2. The log levels set in log_config.yaml don't seem to apply to the cx_logger - info is still the default log level regardless of which level is set in log_config.yaml.