Add unit tests and a gateway component

The Custodian was lacking unit tests. This PR brings unit tests for several components. Note that some functions are not tested because they require communications with other services that are very complicated to mock (mongo, pulsar).

A new folder has also been added: a gateway/reverse-proxy. It contains a reverse proxy that handles user authentication using OpenID connect with Keycloak.

The helm charts have also been fixed to add keycloak secrets as Helm values. The secret-generator has been updated accordingly.