Add signature badge and pipeline status to commit details in MR diff

What does this MR do?

Following up from !21784 (merged), this MR adds the signature badge and pipeline status to the MR diff commit details. This is the last part of #49329 (closed).

It exposes two new properties in the API of diffs.json

• commit.signature_html
• commit.pipeline_status_path

Screenshots

What are the relevant issue numbers?

Closes #49329 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides