SSRF via Unvalidated import-url Artifact URLs

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### Describe the bug

There is a SSRF vulnerability due to unvalidated `import-url` artifact URLs. The `import-url` command accepts arbitrary URLs (e.g., `https://`, `http://`) and passes them directly to the `DownloadArtifact` method. This function sends the URL as a form field (`url`) up to the Microcks API endpoint `/api/artifact/download`. The Microcks server then fetches the URL without sufficient validation, lacking an allowlist, private-IP-range blocking, or scheme restriction.

### Expected behavior

The provided URL should be strictly validated before being fetched by the Microcks server. The validation should deny access to private/internal IP ranges (e.g., `169.254.169.254`, `localhost`, `10.x.x.x`), restrict schemes to only allowed protocols, or enforce an explicit allowlist to prevent arbitrary internal network access.

### Actual behavior

The URL is accepted and processed without validation. When the server fetches the provided URL, an attacker can coerce the server to interact with internal networks. The `secret` field (for repository authentication) may also be passed along, meaning the server could be tricked into sending stored credentials to attacker-controlled external endpoints.

### How to Reproduce?

1. Authenticate with the Microcks CLI (e.g., `microcks login ...`).
2. Run the `import-url` command with a payload targeting an internal network resource or cloud metadata endpoint, such as:
```sh
microcks import-url http://169.254.169.254/latest/meta-data/
```
Or to access an internal service:
```sh
microcks import-url http://localhost:8080/api/admin/
```
3. The CLI passes the URL to the server, and the Microcks server fetches it, acting as an SSRF proxy to the internal resource.

### Microcks version or git rev

master

### Install method (`docker-compose`, `helm chart`, `operator`, `docker-desktop extension`,...)

Any deployment where an attacker has CLI or API access to perform a URL import. Highly critical on Cloud environments.

### Additional information

**Code References:**
- `cmd/importURL.go`: The URL from arguments is handled and directly passed to `DownloadArtifact`.
- `pkg/connectors/microcks_client.go`: `DownloadArtifact` embeds the `url` and `secret` unsanitized in the `POST` form data submitted to the server endpoint.

**Real-World Impact:**
In a shared Microcks instance or CI pipeline, any user with `import-url` capabilities can pivot to internal network resources. In cloud deployments, this opens the door to leaking IAM credentials, probing internal networks, and extracting sensitive API responses.

Start with cmd/importURL.go and pkg/connectors/microcks_client.go, then trace how the URL and secret reach the /api/artifact/download endpoint. Clarify where validation belongs and test the required scheme restrictions and private or internal address blocking; done means arbitrary internal fetches and credential forwarding are prevented.

Written by the indexing model from the issue text.