Add cloudflared_route integration

interfaces/cloudflared_route/v0/README.md

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+# `cloudflared_route`
+
+## Usage
+
+The `cloudflared_route` interface is designed to facilitate information exchange between the cloudflared tunnel configurator charm and the cloudflared tunnel charm.
+
+## Direction
+
+The `cloudflared_route` interface follows a provider/requirer pattern. The requirer is a charm that runs the cloudflared tunnel, and the provider is the charm that supplies configurations for the cloudflared tunnel.
+
+```mermaid
+flowchart TD
+  Provider -- tunnel_token_secret_id,nameserver --> Requirer
+```
+
+## Behavior
+
+The following are the criteria that a Provider and Requirer need to adhere to in order to be compatible with this interface.
+
+### Provider
+
+- Is expected to provide a cloudflared tunnel token via a Juju secret.
+  - The Juju secret ID should be placed in the integration application databag with the name `tunnel_token_secret_id`.
+  - The Juju secret should be readable by the requirer charm.
+  - The Juju secret should contain one field, `tunnel-token`, with the cloudflared tunnel token as its content.
+- Is expected to update the cloudflared tunnel token either by creating a new Juju secret and replacing the Juju secret ID in `tunnel_token_secret_id`, or by updating the content of the existing Juju secret ID.
+- Is expected to optionally include a `nameserver` field in the integration databag.
+  - The `nameserver` field must be an IPv4 or IPv6 address of a DNS nameserver.
+  - The nameserver provided in this field must listen on port 53.
+  - The nameserver provided in this field must be able to resolve public domains.
+  - The nameserver provided in this field must be able to resolve any domain configured as the backend service for the cloudflared tunnel.
+
+### Requirer
+
+- Is expected to run a cloudflared tunnel instance with the provided tunnel token.
+- Is expected to restart the cloudflared tunnel instance with a new tunnel token if the token changes in the integration.
+- Is expected to run the cloudflared tunnel instance using the system's default nameserver if `nameserver` is not provided in the integration.
+- Is expected to run the cloudflared tunnel instance with the specified `nameserver` if it is provided in the integration.
+
+## Relation Data
+
+### Provider
+
+[\[JSON Schema\]](./schemas/provider.json)
+
+The Provider provides the result of the requirer's request. It should be placed in the application databag.
+
+#### Example
+```json
+  {
+    "application-data": {
+      "tunnel_token_secret_id": "secret:csn5caau557j9bojn7rg",
+      "nameserver": "1.1.1.1"
+    }
+  }
+```

interfaces/cloudflared_route/v0/interface.yaml

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+name: cloudflared_route
+internal: false
+
+version: 0
+status: draft
+
+providers: []
+requirers: []
+
+maintainer: is-charms

interfaces/cloudflared_route/v0/schema.py

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+# Copyright 2024 Canonical
+# See LICENSE file for licensing details.
+"""This file defines the schemas for the provider sides of the cloudflared_route interface.
+It exposes the interfaces.schema_base.DataBagSchema subclasses called:
+- ProviderSchema
+
+Examples:
+    ProviderSchema:
+        unit: <empty>
+        app: {
+          "tunnel_token_secret_id": "secret:csn5caau557j9bojn7rg",
+          "nameserver": "1.1.1.1"
+        }
+"""
+
+from pydantic import IPvAnyAddress, BaseModel
+from interface_tester.schema_base import DataBagSchema
+
+
+class CloudflaredRouteProvider(BaseModel):
+    """List statuses for the DNS records informed by the requirer."""
+    tunnel_token_secret_id: str
+    nameserver: IPvAnyAddress | None = None
+
+
+class ProviderSchema(DataBagSchema):
+    """Provider schema for dns_record."""
+    app: CloudflaredRouteProvider