Resolve "Uploader service to store SLSA provenance statements as artifacts"

What does this MR do and why?

References

Why

We are working towards SLSA L3 in &15858 . As a part of Phase 2: Generate provenance statement in control plane, we need to implement an uploader service to store the generated provenance on object storage.

What

This merge request creates a new service, Ci::Slsa::UploadStatementService, to store a provenance statement JSON file as an artifact. This is an interim mechanism that will allow us to provide SLSA attestations for our builds, as part of the &15858 epic.

The provenance statement is generated using the Ci::Slsa::ProvenanceStatement model, introduced by a previous MR. We then leverage UploadedFile and JobArtifact::CreateService in order to create a service and handle edge cases. See &17702 (comment 2564118429)

A subsequent MR will use this service from a worker to automatically populate the required artifacts.

How to set up and validate locally

1. Set up GDK with a runner as described here.
2. Create a sample workflow that generates an artifact. Example below.
3. Observe that it has only two artifacts

4. Ci::Slsa::UploadStatementService.new(build).execute
5. Observe new artifact is created. It can be downloaded and it has the correct contents.

cat .gitlab-ci.yml
build-job:
  stage: build
  script:
    - echo "Hello, $GITLAB_USER_LOGIN!"
    - echo "Hello, $GITLAB_USER_LOGIN!" > test.txt
  artifacts:
    paths:
      - test.txt

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Related to #546153 (closed)