Add SLSA provenance statement capability to Rails backend (job artifact archive only)

Why are we doing this work

As part of implementing SLSA provenance statement generation in the Rails backend (&17702 (closed)), we're making the Rails backend capable of generating a provenance statement for a given Ci::Build model, and to render that statement as a JSON document.

This is NOT a behavior change. Follow-up issues of the same parent epic will cover calling provenance generation from a worker, and uploading the statement to object statemetn.

The scope is limited to job artifact archives. Producing a provenance for the individual job artifacts is out of scope.

Non-functional requirements

Documentation:
- SLSA provenance statement format documented as part of Document GitLab SLSA provenance, link from buil... (#426764 - closed).
- UX to enable provenance statement generation is NOT documented. Right now it's enabled by a feature flag. UX to be defined in [Discussion] UX to enable SLSA provenance gener... (#547903 - closed).
Feature flag (optional):
- Provenance statement generation is behind a feature flag.
- However, the issue is about adding capability to the Rails backend. Behavior doesn't change.
- Update flag rollout issue with flag name: [FF] `slsa_provenance_statement` -- Roll out fe... (#547866)
Performance:
- Check that provenance generation consumes a reasonable amount of resources compared to other tasks happening when a CI/CD job finishes. Example: artifact report ingestion.
- Ensure that complexity is constant or linear. Inputs: number of artifacts, number of CI/CD variables.
Testing:
- Validate output against JSON schema of in-toto statement v1 with SLSA provenance predicate .
- Implement unit tests (rspec).

Implementation plan

This PR will be implemented based on the PoC provided by @darbyfrey here.

Create a new serialization framework for SLSA provenance statements.

This object could later be modified to perform attestations.

Create a model, tentatively Ci::Slsa::ProvenanceStatement.
Make the object JSON serializable. https://api.rubyonrails.org/classes/ActiveModel/Serialization.html
Create new tests for the new model.

Create a new method within `Ci::Build`, tentatively `slsa_attestation`, that returns the `Ci::Slsa::ProvenanceStatement` that corresponds to it.

The new endpoint would be accessible as follows: Ci::Build.find(1337).slsa_attestation.

Create a method within the Ci::Build model.
Ensure the method populates the field as documented in this related issue. #536022 (closed)
Create unit tests for the new method.

Verification steps

Manually trigger a local build and inspect the attestation object by interacting with Ci::Build.to_slsa.
Ensure documentation is ready to go #426764 (closed)