Add option to customise webhook timeout.

### Problem Statement

Webhook timeout is currently 1s, which is too short for certain scenarios, leading to a large number of timeouts.

### Solution Brainstorm

Add the option for users to increase this timeout, or set it higher by default for all users.

[Original request.](https://sentry.zendesk.com/agent/tickets/173245)

### Product Area

Settings - Integrations

The issue identifies Settings - Integrations as the product area but names no files or tests. Start by locating the webhook timeout implementation and the integration settings entry point; determine the supported configuration scope and default behavior, then add coverage showing the configured timeout is honored.

Written by the indexing model from the issue text.