Add Bearer token authorization expiration strategies

[gregglind]

Is your feature request related to a problem? Please describe.

api-client HeaderAuthentication assumes that all Bearer tokens are valid.

When these token expire or are invalidated at the server,

Describe the solution you'd like

1. Add authenticate or bearer_authenticate method that defines how to get a valid Bearer token. Bonus: This method should implement backoff and error-handling.

Describe alternatives you've considered

1. Define and a BaseAuthentication class to contain the methods for getting an authorization.

Additional context
This is a hard request, and I am very interested in hearing what designs sound good to others.