Point OAuth URLs to Jira connect instance_url

What does this MR do and why?

For Jira app installations that point to a self-managed instance instead of SaaS, we want to initialize the OAuth flow for the instance. Therefore, the authorize and token URL need to point to the instance.

• oauth_authorize_url will be used as the entry point for the OAuth flow. The user will be redirected to this URL after clicking the login button.
• oauth_token_url will be used to fetch the access token after a successful OAuth authentication.

Related issue: #373029 (closed)

Screenshots or screen recordings

How to set up and validate locally

The OAuth flow can be tested using this instructions. Testing the self-managed flow is behind the jira_connect_oauth_self_managed feature flag. It's currently not guaranteed that it can be tested because there is still work to do on frontend and backend. We will do an intensive manual QA once everything is done.

You can follow the ongoing work in the related epic: &5650 (closed)

MR acceptance checklist

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