[6.18.z] Add entity for content view environments endpoint

[Satellite-QE]

Cherrypick of PR: #1357

This PR adds an entity class for the content view environments endpoint. This is required for automating SAT-34301.

Upstream API documentation, plugin, or feature links

https://apidocs.theforeman.org/katello/4.18/apidoc/v2/content_view_environments/index.html

Demonstration

PRT run in SatelliteQE/robottelo#19581.

[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location> `nailgun/entities.py:3269` </location>
<code_context>
+        """Get the list of content view environments, passing along any query parameters."""
+        kwargs = kwargs.copy()
+        kwargs.update(self._server_config.get_client_kwargs())
+        url = f'{self._server_config.url}/{self._meta["api_path"]}'
+        response = client.get(url, params=params, **kwargs)
+        return _handle_response(response, self._server_config, synchronous, timeout)
</code_context>

<issue_to_address>
**suggestion:** URL construction may result in double slashes if server_config.url ends with a slash.

To prevent double slashes, use urljoin or remove any trailing slash from self._server_config.url before concatenation.

Suggested implementation:

```python
        from urllib.parse import urljoin
        url = urljoin(self._server_config.url, self._meta["api_path"])
        response = client.get(url, params=params, **kwargs)
        return _handle_response(response, self._server_config, synchronous, timeout)

```

If `urljoin` is already imported elsewhere in the file, you can remove the inline import and use the existing one.
</issue_to_address>

### Comment 2
<location> `nailgun/entities.py:3268` </location>
<code_context>
    def list_content_view_environments(self, params=None, synchronous=True, timeout=None, **kwargs):
        """Get the list of content view environments, passing along any query parameters."""
        kwargs = kwargs.copy()
        kwargs.update(self._server_config.get_client_kwargs())
        url = f'{self._server_config.url}/{self._meta["api_path"]}'
        response = client.get(url, params=params, **kwargs)
        return _handle_response(response, self._server_config, synchronous, timeout)

</code_context>

<issue_to_address>
**suggestion (code-quality):** Merge dictionary updates via the union operator ([`dict-assign-update-to-union`](https://docs.sourcery.ai/Reference/Default-Rules/suggestions/dict-assign-update-to-union/))

```suggestion
        kwargs |= self._server_config.get_client_kwargs()
```
</issue_to_address>

---

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[sourcery-ai]

suggestion: URL construction may result in double slashes if server_config.url ends with a slash.

To prevent double slashes, use urljoin or remove any trailing slash from self._server_config.url before concatenation.

Suggested implementation:

        from urllib.parse import urljoin
        url = urljoin(self._server_config.url, self._meta["api_path"])
        response = client.get(url, params=params, **kwargs)
        return _handle_response(response, self._server_config, synchronous, timeout)

If urljoin is already imported elsewhere in the file, you can remove the inline import and use the existing one.

[Gauravtalreja1]

PRT passed in SatelliteQE/robottelo#19604