[QE] Convert calls to webhook endpoint to use api client

The webhooks endpoint is missing from the api spec for changelog. This is preventing us from implementing the requests via an api rest client. We currently do this in tests via curl. We need to add the spec for the endpoint to the api spec and update the function to send_webhooks. If someone on the dev team updates the api spec, we will only need to re-gen the api and update the send_webhooks function.