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]

Suggested change

	except OpenEoApiError:
	pass
	except OpenEoApiError as e:
	warnings.warn(f"Unable to load the OpenID Connect provider list: {e.message}")

[soxofaan]

we try to avoid usage warnings (I see we still have some cases of that in connection.py), as it generally composes not that good with other tooling. Instead use _log.warning().

Also when including the exception in the warning message, we typically just do {e!r} so that more useful info is included (error code, http code, error message, correlation id )

so:

_log.warning(f"Unable to load the OpenID Connect provider list: {e!r}")

(!r is a shortcut for generic repr()-style rendering of the exception)

[m-mohr]

Added a warning for now.

Must say I don't like the {e!r} though.

Showing in a GUI (like QGIS, not Python coding)

Unable to load the OpenID Connect provider list: OpenEoApiError('[500] Internal: Maintanence ongoing')

feels like bad UX compared to:

Unable to load the OpenID Connect provider list: Maintanence ongoing')

But if the former is consistently done everywhere, I guess that's how it shall be for now...