Add pod annotations for jobs

This adds pod annotations for jobs. This makes it possible to run CI jobs for example with an AWS IAM role annotation.

Configurable with runners.podAnnotations. Example:

runners:
  podAnnotations:
    iam.amazonaws.com/role: my-iam-role

Note: this is not the same as !44 (merged), as that adds annotations to the gitlab runner itself, this MR does that for job pods.

Fixes gitlab-org/gitlab-runner#4050.