Revisit Git repository creation flow credential setup

Scope

• Drop down with existing credentials & add button to the right.
• If no existing credentials, show "none" in dropdown
• Clicking add launches the modal to add credential.
• Once credential is added, modal is dismissed & the credential that was just added populates the dropdown
• Do not provide the "Use system ssh" option - remove it for now and it can be revisited later if there are too many complaints
• Integrate eager validation as described in JENKINS-41597 into this ticket
• Integrate showing/hiding credentials dropdown and "add" button based on credential permissions JENKINS-41573

Design brief
We need a better design for using existing credentials or creating a new credential. I realise this was my original design but it is confusing.

Perhaps we ask them to choose between a new credential or existing one then show the form to select/create a new credential depending on the option picked.

Should we show "existing credential" if there there are no existing credentials to pick from?