Add support for authentication via JWT token

[joemcdonnell]

Impala recently added support for authentication via a JWT token in IMPALA-10489. This allows a user to authenticate with a separate trusted authentication service, obtain a JWT, and pass that JWT to Impala on the Authencation Bearer HTTP header.

This implements support for connecting with auth_mechanism=JWT, which requires a 'jwt' argument specifying the standard string encoding for the JWT. This is passed on the Authentication Bearer HTTP header on each message.

Fixes #462

Testing:

• Added unit tests in test_dbapi_connect.py
• Tested against a running Impala cluster with JWT authentication

[joemcdonnell]

Thanks for the review!