Migrate loading of the API spec out of the ZAP scripts

What does this MR do?

Adds APISpecification to load the API specification from the given URL or file during a DAST API scan. This is prerequisite work for migrating the active scan logic.

NOTE: the API spec loading has not yet been removed from the ZAP scripts and the loading in DAST code is deliberately disabled until it is removed from ZAP. This will happen in a future MR that migrates the active scan logic. DAST code will not use the API spec until then.

What are the relevant issue numbers?

gitlab-org/gitlab#233832 (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