Extract logic to prepare DAST job variables into separate service

What does this MR do?

This change extracts logic responsible for preparing DAST service params and job variables into separate services so they can be used in different context.

This MR was extracted from WIP MR with all needed changes: !54067 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry

Not needed:

Any developer-facing change (e.g., refactoring, technical debt remediation, test suite changes) should not have a changelog entry. Example: “Reduce database records created during Cycle Analytics model spec.”

• 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

Related to #299215 (closed)