Add a mix task to easily generate JWT tokens

[achouippe]

By using authentication settings defined in the neurow configuration, this mix task prints JWT tokens on the standard output that are accepted by the public and internal APIs,

Public API

Generate tokens for the public API:

mix generate_jwt_token --api=public --issuer=test_issuer1 --topic=user:1234

Then to subscribe to the public api with curl:

curl http://localhost:4000/v1/subscribe -H "Authorization: Bearer <token>"

Internal API

Generate tokens for the internal API:

mix generate_jwt_token --api=internal --issuer=test_issuer1

Then, to publish messages on the internal api with curl:

curl http://localhost:3000/v1/publish -H "Content-Type: application/json" -X POST -d '{"message": "Hello world!" , "topic": "user:1234"}' -H "Authorization: Bearer  <token>"