Add URL support for FileContent

Checklist

• I have read both the CONTRIBUTING.md and Contributor License Agreement documents.
• I have created an issue or feature request and received approval from xAI maintainers. (minor changes like fixing typos can skip this step)
• I have tested my changes locally and they pass all CI checks.
• I have added necessary documentation or updated existing documentation.

Description

Adds SDK support for the new url field on FileContent in the proto, allowing users to attach files via public URL in addition to the existing file_id and inline data modes.

Related Issue

N/A.

Type of Change

• Bug fix
• New feature
• Documentation update
• Other (please specify)