Create UI for logging in to a cluster

Create a webview for logging into a cluster. This should be a form where:

1. The user selects the cluster to log in to. They should have the option to pick an existing cluster or add a new cluster using its URL
2. If necessary, the user provides authentication for the cluster. This could be:
   a. username and password
   b. token

This workflow should be similar to the existing "Login" workflow that uses quick picks.

The motivations for doing this are:

• Currently, the user can't go back after selecting the cluster that they want to log in to in the case they selected the wrong cluster

I personally disagree with making this change, since it will add additional complexity to maintaining and testing the extension, with little added benefit to the end user.