Add OAuth flow for self-hosted on Jira connect app

What does this MR do and why?

Note: This MR is split from the original changes in !89378 (closed) and is behind a feature flag jira_connect_oauth_self_managed.

Adds the remaining steps to enable the OAuth flow for self-managed:

• When user selects version (GitLab.com or self-hosted) and clicks save, an API request is made to update the Jira connect installation on GitLab.com to include the instance_url.
• If successful the page reloads so the backend can include the instance_url in the Content Security Policy.
• Add remaining code for OAuth URL generation and launching a new window.

Here is a list of MRs in this series.

MR	Status
Refactor API calls to support self-hosted on Jira connect app	✅ Merged
Add UI support for self-hosted on Jira connect app	✅ Merged
Add OAuth flow for self-hosted on Jira connect app	👈 You are here

Screenshots or screen recordings

No changes to current UI but you can preview the new one in this video https://www.youtube.com/watch?v=YRdDM04hy6E.

How to set up and validate locally

See the original MR description for video run-through and detailed steps: !89378 (diffs)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #356361 (closed)