Draft: POC for polling CSV export

What does this MR do?

This is a POC MR to evaluate polling for CSV export.

When a merge commit csv is requested, a background job is created that computes, generates CSV and maintains the status of the export. The UI meanwhile polls to see if an export is ready, and shows the download button accordingly.

Improvements:

• Poll only when there is an ongoing export in process
• Disable new report generation when there is an ongoing export
• Provision to cancel an ongoing export and generate a new one

csv-polling-demo

Screenshots

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