[Tunnelbroker] implement OAuth 2.0 JWT token encoding

Summary:
Implementing what is described in Google's OAuth 2.0 for Server to Server Applications, Forming the JWT claim set.

get_auth_bearer is only public method, will handle generating and refreshing token.

Depends on D12753

Test Plan:
Call get_jwt_token and test if returns the correct JWT token in the format

{Base64url encoded header}.{Base64url encoded claim set}

Later in the stack this token is used to make an HTTP call to get an access token for the FCM push service.

Reviewers: bartek, marcin

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12754