Add or update Python client constructor to streamline in-cluster initialization

[deliahu]

Description

There should be a simple way for users to in initialize the Cortex Python client from within their Predictor implementation.

Motivation

This would be useful for tasks such as updating an API after a batch job finishes

Implementation

client() would not have to take in an env name, and when not provided, it will use the default environment. The operator would mount a configmap and set CORTEX_CLI_CONFIG_DIR to point to it