Configurable Base URL for the Jira Cloud App

Proposal

When integrating GitLab with Jira Cloud behind a reverse proxy, there's currently a limitation where the external URL (reverse proxy FQDN) is used for branch creation operations.

As noted in the documentation: https://docs.gitlab.com/administration/settings/jira_cloud_app/#using-a-reverse-proxy

When you select Create branch in the Jira development panel, you are redirected to the reverse proxy FQDN rather than the internal GitLab FQDN.

This poses a problem in some organizations, where GitLab should only be accessible via the internal GitLab FQDN. The current implementation causes the Jira integration to use the external URL: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/controllers/jira_connect/app_descriptor_controller.rb?ref_type=heads#L139

We could add a configuration option that allows administrators to specify the base URL to use specifically for Jira Cloud App integration, separate from the general external URL configuration. This would enable organizations to:

• Use their internal GitLab URL for branch creation operations when accessed from behind the proxy
• Maintain the external URL for access from outside the network
• Support proper functioning of the Jira Cloud integration regardless of network configuration