Create an inteface for sharing Dex's `issuer` value

[DnPlas]

Context

In order to make it easier to configure the OIDC provider in both Dex and OIDC clients (e.g. oidc-gatekeeper), this charm should have the capacity of broadcast its issuer value to OIDC client charms that would like to integrate to it.

The interface schema should be similar to:

# provider side
{
  # metadata
  ...
  "required": {
    "dex-issuer-url",
  },
  "properties": {
    "dex-issuer-url" {
      "type": "string",
      "description": "This is the canonical URL that OIDC clients will use to refer to dex"
    },

  }
}

# requirer side does not share data, so the schema is only metadata like description and name

For now the issuer-url will be the only shared value, but the library can be expanded for more.

What needs to get done

1. Create a library that handles the interface
2. Add the relation to the dex-auth charm
3. Add unit/integration tests as required

Definition of Done

This charm broadcasts its information (for now limited to just the issuer) regardless if an OIDC client is integrated.

[syncronize-issues-to-jira[bot]]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5966.

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