Make backend aware of incremental rollout

During %11.4 planning with struggled with the backend lack of knowledge about an ongoing incremental rollout.

We may think about a variation of gitlab-foss!21554 (closed) that will introduce a new syntax for gitlab-ci.yml

deploy:
  environment:
    name: production
    rollout: 10

And records rollout and the deployment track in a new model Ci::BuildDeployment, so that we can keep track of scheduled deployment as well.

This feature will allow implementing gitlab-org/gitlab-ee#5417 and gitlab-org/gitlab-ee#5416 will also boost our ability to cross-reference ~"devops:monitor" and ~"devops:release" informations allowing future development like self-healing/auto-rollback deployments.

/cc @jlenny @erushton