Create a first-class HTTP message class

What does this MR do?

Moves src/scanned_resources to src/models/http/http_messages. This was requested/discussed in an MR review.

This MR also:

• Uses the HTTPMessage in ZAProxy and associated report formatters
• Converts the MessageResultsParser to return HTTPMessages
• Creates factories for each new model type

What are the relevant issue numbers?

gitlab-org/gitlab#231326 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer