Add setting and implement code changes to run in traction mode

[esune]

When running in traction mode, the controller will need to authenticate to the tenant using wallet_id/wallet_key and exchange them for a bearer token to be used as means of authenticating with the agent.

#74 demonstrates how to retrieve the token given the appropriate id/key combination and submit a request to the tenant. The set-up part creating a traction tenant and setting the webhook endpoints, however, is not necessary as the expectation is that the tenant will be provisioned separately and will be ready to be used.

The behaviour should be driven by a global setting ISSUER_MODE that will take values standalone or traction, switching the authentication behaviour to the agent accordingly.