refactor(1.4): Make the cloud client APIs more structured and aligned

[frostming]

Signed-off-by: Frost Ming me@frostming.com

What does this PR address?

Make the cloud client APIs more structured and aligned

import bentoml
import os

# Use default client
deployment = bentoml.deployment.get('mydeployment')
# Use from cloud client
client = bentoml.BentoCloudClient("token")
deployment = client.deployment.get("mydeployment")
# Env vars are supported
os.environ['BENTO_CLOUD_API_KEY'] = 'mytoken'
os.environ['BENTO_CLOUD_API_ENDPOINT'] = 'https://my.cloud.bentoml.org'
client = bentoml.BentoCloudClient()
deployment = client.deployment.get("mydeployment")