[Tunnelbroker] add jsonwebtoken dependency

Summary:
This library implements creating and encrypting JSON web token.

Link to create: docs.rs.

Apple docs and spec: link.

Used in D12610

Depends on D12606

Test Plan: N/A

Reviewers: bartek, marcin, ashoat

Reviewed By: bartek, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12607