Trace: Implement V2 API client

[liyanhui1228]

As the Stackdriver Trace V2 API is launched, we need to implement client library to support using it.

Checklist for this issue:

• Merge the Autogen Trace V2 library (PR Auto-generate Trace v2 #4379)
• Implement Stackdriver Trace V2 API client

After the Trace V2 API client library is available, calling from google.cloud import trace will by default using the V2 API. To use V1 API, one will need to explicitly import like this: from google.cloud.trace.v1 import Client.