Add a tool to synchronize webhooks on our repositories

[pietroalbini]

This PR adds a tool that synchronizes the webhooks configured in a repository with the configuration stored on this repo.

The tool is located in tools/sync-webhooks and uses the configuration in config/repo-webhooks. It supports presets to share common webhooks (like bors) between multiple repos, dry runs to preview the changes that are going to be applied and synchronizes as much settings as it cans with the constraints we have:

• Due to GitHub API limitations it's not possible to update the content type or the required SSL once the webhooks is created: doing so will reset the secret key, and there is no way to avoid that
• Secrets are not set by the tool, but it will post a link in its output to the hook's settings page, where a person can set the right one

At the moment I only configured rust-lang/rust, and we can add new ones once the PR is merged.

r? @aidanhs @alexcrichton