Add http agent support to JWT Auth

[mutaphysis]

Expected Behaviour

JWT Auth works with custom http agent to support eg. proxy servers or custom dns resolving

Actual Behaviour

JWT uses node-fetch without any means to configure the http agent

Description

Some IT environments have additional requirements such as forcing users to use a http proxy with authentication or having other custom requirements. For this node-fetch foresees passing in an agent option, that allows custom network handling. See https://github.com/node-fetch/node-fetch#custom-agent https://nodejs.org/api/http.html#http_class_http_agent .

I propose extending the options of auth to allow setting an agent.

framefield@7e57f0d

[macdonst]

Seems like a good candidate for a PR @mutaphysis

[mutaphysis]

Thanks, I opened #51