Add jira_connect_proxy_url application setting

What does this MR do and why?

This is part of #372967 (closed). See !96818 (closed) for a full context MR.

When a user installs the GitLab for Jira app we receive an installed hook. It includes a JWT token that we have to verify using a public key. The public key is fetched from connect-install-keys.atlassian.com (see lib/atlassian/jira_connect/jwt/asymmetric.rb:15).

To make the app available for self-managed users, GitLab.com will serve as a proxy. It forwards the installed hook to the self-managed instance, but generates a new JWT token. To make this work, we need to:

1. Build the JWT infrastructure (!98431 (merged))
   • Add a service to generate JWT tokens.
   • Store the public keys with an expiry date.
   • Provide an endpoint to fetch public keys.
2. Allow the public key CDN URL to be configured. (This MR)
   • Add an application setting that defaults to https://connect-install-keys.atlassian.com and can be pointed to https://gitlab.com/-/jira_connect/-/jira_connect/public_keys.
3. Forward the installed event to self-managed
   • Add a service that sends an installed hook to the self-managed instance when instance_url is updated.

I explained the problem in more detail in #372967 (closed)

How to set up and validate locally

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