Function to fetch and refresh tokens

[SBlechmann]

Is your feature request related to a problem? Please describe.
Currently, FiLiP only offers a way to handle Bearer tokens in the header but there is no function or example how to fetch (and refresh) a token.
Imo, we should provide a function that does so and adjust the example.

Describe the solution you'd like
Implement our internal functions for the keycloak_token_handler in FiLiP.

[djs0109]

Good points. I would integrate it as an example that works for now. There are a lot of third-party libs that can authenticate the session/client/etc, that can be used by FiLiP clients. Therefore, it will be a bit redundant for FiLiP to support and maintain this functionality.

[github-actions]

Branch 316-Function-to-fetch-and-refresh-tokens created!