Implement list_auth_providers

[m-mohr]

This implements a new method list_auth_providers so that clients can get a full list of authentication methods/providers easily.

Planned to be used in e.g. the openEO QGIS plugin, which plans to mimic the authentication interface of the Web Editor:

For this, we need a way to retrieve all auth methods. We could also have this code in the plugin itself, but it felt better suited here. Considering also, that the other clients have similar functions.

[soxofaan]

instead of using separate asserts, one for each fields, I prefer a single assert that checks the response as a whole, which is perfectly doable here, e.g.

assert providers == [
    {
        "type": "oidc", 
        "issuer": ...
        "title": ...
    },
    {
         ...
]

The advantage of this is that you can easily see and understand the expected output.
And when there is mismatch, pytest will visualize the difference in a very accessible way (when verbosity level is high enough, e.g. see https://docs.pytest.org/en/stable/how-to/output.html#verbosity)

[m-mohr]

I wasn't sure whether this might lead to problems when the (stub) API (for whatever reason) would change the order of the array?!

[m-mohr]

Is anything missing for this getting merged?