Expose DAST request and response evidence

What does this MR do?

The DAST scanner outputs in the JSON report details of the request and response that was made when a vulnerability was detected. This context is useful for users when they review vulnerabilities as it can help them determine whether or not the vulnerability is a false positive.

This MR exposes the request and response evidence so that the Rails frontend can consume it and display it on the screen.

This MR is part of issue #215679 (closed). (parent issue: #36332 (closed)) (frontend issue: #215110 (closed))

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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