Add `authorization_endpoint` field in Dex's configuration

### Context

Adding the `authorization_endpoint` field in the [configuration](https://github.com/canonical/dex-auth-operator/blob/main/src/charm.py#L228) will allow OIDC clients, such as oidc-gatekeeper, to automatically know the endpoint they should use for starting the [Authorization Code Flow](http://openid.net/specs/openid-connect-basic-1_0.html#CodeFlow).

This is useful because by having this information available in Dex's `.well-known` endpoint will allow reducing the required configuration for oidc-gatekeeper, as it will automatically infer it.

### What needs to get done

1. Add `"authorization_endpoint": "/dex/auth"`as part of [these lines](https://github.com/canonical/dex-auth-operator/blob/main/src/charm.py#L228C1-L241C10)

### Definition of Done

The `.well-known` endpoint shows this information and OIDC clients can use it. To verify this we can `curl -v /dex/.well-known/openid-configuration`

Open src/charm.py around lines 228-241 and inspect how Dex's .well-known configuration is assembled. Verify the completed metadata with curl -v /dex/.well-known/openid-configuration; done means the response includes authorization_endpoint and OIDC clients can infer it.

Written by the indexing model from the issue text.