Add policy for checking insecure_ssl configuration for github_organization_webhook in GitHub provider

[HorizonNet]

The GitHub provider has a resource github_organization_webhook, which can be used to setup a webhook for an organization. In the configuration it allows several options. One of the them is insecure_ssl, which is not recommended to be set to true. The new policy for the github policy set should check this configuration and report a violation if this option was set to true.