Jira Server Integration - allow implementing without Jira Admin RSA Key

### Problem Statement

Currently sentry's Jira server integration requires admin access to the jira server so that it can create the necessary webhooks. In a lot of environments getting this access is not possible since teams won't give Jira admin rights for external integrations due to (reasonable) infosec concerns.
This was shared in a forum a couple years ago however never turned into anything.
https://forum.sentry.io/t/oauth-against-jira-server/7594/10

The image below occurs when attempting to add the integration without admin access:

### Solution Brainstorm

Make the automatic creation of the webhook optional and provide some instructions for how to create the webhook manually. It should be easy enough to copy a URL into the JIRA webhook field.