Fetch & Configure Secrets with HashiCorp Vault
### Phase 2 - Fetch & Configure Secrets
* [ ] [GitLab reads Vault CI Variables](https://gitlab.com/gitlab-org/gitlab/-/issues/28321)
* https://gitlab.com/gitlab-org/gitlab/-/merge_requests/31831 ~"workflow::In review" Define new CI syntax for `secrets`.
* https://gitlab.com/gitlab-org/gitlab/-/merge_requests/32501 ~"workflow::In review" Expose configured secrets (if any) to the runners, apply license check. Currently behind feature flag, disabled by default.
* TBC - add support for defaults in the CI syntax, as proposed here - https://gitlab.com/gitlab-org/gitlab/-/issues/28321#defaults
* [ ] [Fetch requested secrets from Vault and provide them to CI jobs](https://gitlab.com/gitlab-org/gitlab/-/issues/212252)
* [ ] [Explicit concept of secrets in GitLab CI](https://gitlab.com/gitlab-org/gitlab/-/issues/218746)
* [ ] Document Vault backend CI secrets (i.e. how to configure your Vault)
* [ ] [ ] Project level Vault Configuration
* [ ] Configure secrets as CI variables
* [ ] Add support for jobs to require secrets in CI syntax
* [ ] [Alternatively] Configure secrets in CI syntax
* [ ] Fetch requested secrets from Vault and provide them to CI jobs
* [ ] Limit communication with Vault (add timeout)
* [ ] Document Vault backed CI secrets (i.e. how to configure your Vault)
epic