Extract integration custom fields to Vue

What does this MR do?

Context: This is in a series of MRs to move / refactor this form from HAML to Vue. The most recent one !31074 (merged) was just merged moved the trigger events section.

This MR builds on top of that and moves the dynamic fields section (which is the rest of the form). Specifically:

• On the backend:
  • Add a new serializer to pass data to the frontend (as suggested in the previous MR)
  • Reuse the integration_form_refactor feature flag to hide the form until all work is done.
• On the frontend:
  • Add a new DynamicField component which renders different type of input based on what is sent from the backend.
  • Pass data from backend to frontend in HAML view via data attributes.

This will be followed by at least 1 more MR to adjust the ActiveToggle and JiraTriggerEvents components (alignment of label from side-by-side to label above input) before we enable the feature flag.

Screenshots

Integration	Before (without ff enabled)	After (with ff enabled)
Slack notifications

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