Allow CI Reports to be Namespaced to support monorepos
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### Release notes
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### Problem to solve
<!--What problem do we solve? Try to define the who/what/why of the opportunity as a user story. For example, "As a (who), I want (what), so I can (why/value)."-->
In a monorepo, there is a need to segment aspects of scans and test reports based on parts of the repository (whether that is a specific file or application) rather than inherently all of the contents of the repository. In GitLab, reports are currently tied to artifacts and the artifacts are tied to the gitlab-ci.yaml file which is tied to the project, so this means that reports are viewed at the whole entirety of the project or source code repo.
As GitLab is leveraged for more diverse source code and CI needs, we are missing the ability to scope CI Report Artifacts within specific areas of a repository.
Currently all results for a given "report type" (i.e. `sast` or `junit` or [many others](https://docs.gitlab.com/ee/ci/pipelines/job_artifacts.html#artifactsreports)) get merged into the same dataset. This is convenient when I want MANY jobs to funnel into a single report, but for when I want to have different Reports for different areas within a given codebase, I am unable to do so.
For DAST, oftentimes the "Default Branch" will be used to deploy to multiple environments and if multiple environments run DAST scans, the results will be a combination of all environments - which is not helpful.
### Intended users
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead)
* [Sam (Security Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sam-security-analyst)
* [Simone (Software Engineer in Test)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#simone-software-engineer-in-test)
### User experience goal
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We ought to support namespaces for Artifact Reports
### Proposal
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Add a `namespace` designation so that reports when defined can be namespaced.
```yaml
sast:app1:
variables:
CI_PROJECT_DIR: app1/
artifacts:
reports:
sast: gl-sast-report.json
namespace: app1
sast:app2:
variables:
CI_PROJECT_DIR: app2/
artifacts:
reports:
sast: gl-sast-report.json
namespace: app2
```
**OR** like how [environment namespaces](https://docs.gitlab.com/ee/ci/environments/#grouping-similar-environments) work:
```yaml
sast:app1:
variables:
CI_PROJECT_DIR: app1/
artifacts:
reports:
sast/app1: gl-sast-report.json
sast:app2:
variables:
CI_PROJECT_DIR: app2/
artifacts:
reports:
sast/app2: gl-sast-report.json
```
**OR - leverage CODEOWNERS Sections (thanks @dzalbo) for the idea.**
Augmented by "labelling' vulnerability objects with [section names from CODEOWNERS](https://docs.gitlab.com/ee/user/project/codeowners/#organize-code-owners-by-putting-them-into-sections). A lot of customers using monorepos already employ CODEOWNERS as a solution to set up ownership and approval rules based on which component was modified. We could apply the same logic and automatically categorize vulnerabilities (at least SAST and SCA as they can be tracked to specific location within repository). This way there will be no need to set up namespaces in CI and the solution would also work well with incremental SAST scanning ([Incremental SAST scanning (only scan code chang... (#419734)](https://gitlab.com/gitlab-org/gitlab/-/issues/419734 "Incremental SAST scanning (only scan code changed in the MR)"))
### Further details
<!--Include use cases, benefits, goals, or any other details that will help us understand the problem better.-->
This is vital for to enable support for monorepo architectures.
### Permissions and Security
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### Documentation
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### Availability & Testing
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