Integrations: support self-hosted providers from gkdev

[d13]

The goal is not to just convert over to GKdev like our other providers. We need to continue support of multiple instances of self-hosted and GKdev only supports one instance.

High level changes:

• ✅ Send users to GKdev to connect to self-hosted GitHub or GitLab
  • ✅ if the integration exists on GKdev show the enable/disable toggle on the remote
  • ❌ if the integration does not exist: show the enable toggle and send user to GKdev
  • ✅ Show the "Connect GitHub Enterprise" on the Home View
• ✅ Existing self-hosted integations (GitHub or GitLab) on GKdev, bring those into our system, but still support our custom self-hosted auth
• ✅ Once we have the info, we need to register a custom provider
• ➡️ A follow-up (Launchpad): Support GitHub Enterprise (GKDev) in Launchpad #3923
• ➡️ Another follow-up (GitLab): Support GitLab Self Managed integration from GKDev #3934

👍 The code where this happens in remoteProviders.ts
👍 Essentially, we can just take the list of custom remotes and add another custom entry from the GKdev data, at runtime. We don't want to update their settings.json (though it might be an easy path for initial testing).

Potential gotchas:

• During development, if GKdev doesn't provide the correct data, let's just temporarily stub the data in while we wait for GKDS/GKdev to provide the info (aka URL)

Testing

• ✅ issue linking works
• ✅ pull requests work