Allow delivery-metrics to receive webhook events from deployment pipeline projects

What does this MR do and why?

Describe in detail what your merge request does and why.

• Add a handler for the job webhook API.
• Currently, 2 webhook routes are added, both for job event webhooks from 2 projects. Separate routes per project allow us to know exactly which token to expect without parsing the body.
  • Route /webhooks/job/ops/gitlab-org/release/tools for receiving job event webhooks from https://ops.gitlab.net/gitlab-org/release/tools/.
  • Route webhooks/job/ops/gitlab-com/gl-infra/deployer for receiving job event webhooks from https://ops.gitlab.net/gitlab-com/gl-infra/deployer.
• Webhook API calls contain a token in a request header. The token is checked against an environment variable. Each project uses a different token.
• Parse the webhook event in JSON format received from a webhook API call, using the github.com/xanzy/go-gitlab library.

This MR will make it easier to use job event webhooks to update deployment pipeline related metrics like "number of manual job retries". In a follow-up, we will start adding metrics to be updated using the data received by the webhook handlers.

gitlab-com/gl-infra/delivery#2708 (closed)

Author Check-list

• Has documentation been updated?