[bug]: Incoming Webhook 'Avatar URL' field has no validation

[PrinceGautam2106]

Summary

This PR adds validation for the avatar_url field in incoming webhook configurations to ensure that only valid URLs are accepted.

Currently, the avatar_url field accepts any string value. This allows invalid inputs such as:

avatar_url: "abc123"

Invalid URLs can lead to UI rendering issues when Rocket.Chat attempts to display the avatar associated with webhook messages.

Impact

• Improves validation for webhook payloads.
• Prevents malformed avatar_url values from affecting UI rendering.
• Maintains backward compatibility for valid configurations.