Add webhook secrets for SDK experiment triggers

[BBYNAI]

Describe the feature or potential improvement

Description

Langfuse supports triggering SDK experiments from the UI by configuring a webhook URL that gets called when running datasets via SDK/API. However, unlike prompt automation webhooks which now support webhook secrets for signature verification (added in #7807), the SDK experiment trigger webhook does not have this security feature.

Proposed feature

Add webhook secret support for the "Trigger SDK Experiment from UI" feature that:

• Allows users to configure a webhook secret alongside the webhook URL
• Signs outgoing webhook requests with the secret
• Enables receiving servers to verify the webhook signature to ensure requests originate from Langfuse

This should mirror the implementation added for prompt change webhooks in #7807.

Additional information

No response