Vulnerabilities API now takes optional pipeline_id

What does this MR do?

Adds pipeline_id as an optional parameter for the Vulnerabilities API.

This change allows the backend to generate the vulnerability reports limited to the given pipeline, which is functionality needed for the following issues:

https://gitlab.com/gitlab-org/gitlab-ee/issues/11975, https://gitlab.com/gitlab-org/gitlab-ee/issues/11976, https://gitlab.com/gitlab-org/gitlab-ee/issues/11978, https://gitlab.com/gitlab-org/gitlab-ee/issues/11979

The frontend change to start consuming this change to the API will be behind a feature flag.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.