Add dex-issuer-url and remove public-url configuration options

[DnPlas]

Context

Because the issuer URL in Dex's configuration could be either the application's Kubernetes Service DNS name + the dex endpoint (e.g. http://<dex-auth-app-name>.<namespace>.svc:5556/dex.) or a publicly accessible IP/DNS name (e.g. http://foo.com/dex), this charm should be configurable for the latter case.

This task also requires the removal of the public-url configuration option as it has been considered misleading and should not be used. Please refer to canonical/bundle-kubeflow#608 for more information.

For details about the design, please refer to this spec (internal only).

What needs to get done

1. Remove the public-url configuration option
2. Add the following configuration:

# dex-auth config.yaml 
 dex-issuer-url:
	type: string
	default: ""
	description: |
	  Format: http(s)://<publicly-accessible-dns-name>/dex
          (Also referred to as issuer) This is the canonical URL that OIDC clients MUST use to refer to dex. If not specified, it defaults to dex-auth's local endpoint constructed from dex-auth's Kubernetes Service DNS name, the Service port and Dex's endpoint, that is http://<dex-auth-app-name>.<namespace>.svc:5556/dex.
          The default is set by the charm code, not the configuration option.
          This configuration must be set when using a Dex connector that will try to reach Dex from outside the cluster, thus it should be a publicly accessible endpoint, for example:
          https://my-instance.in-my-cloud.some-cloud.com/dex

3. Remove any traces of the "public_url" value from the charm code
4. The issuer value will be set to either the default http://<dex-auth-app-name>.<namespace>.svc:5556/dex or model.config["dex-issuer-url"]
5. Add/modify tests appropriately

Definition of Done

The charm now gets the issuer value from either the configuration option or defaults to http://<dex-auth-app-name>.<namespace>.svc:5556/dex. This can be confirmed by checking the .well-known endpoint.

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5967.

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[DnPlas]

Fixed by #209