Expose commit sha on Vulnerabilities::Occurrence

What does this MR do?

As reported #13857 (closed) presenter for Vulnerabilities::Occurrence doesn't generate blob_path if sha is missing which causes occurrence widget to not generate links for affected files. This MR adds sha accessor on Vulnerabilities::Occurrence. SHA will be sourced from pipeline.sha for PipelineVulnerabilitiesFinder and from joined table for VulnerabilityFindingsFinder (via Vulnerabilities::Occurrence.for_pipelines_with_sha)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team