Support selecting an existing profile for IdC login flow

[breedloj]

Currently it is only possible to create a net new login when using the Pro/IAM Identity Center based login flow. Other IDE plugins, such as Amazon Q for Jetbrains, support the ability to re-use an existing profile to make this connection. This would optimize the login flow for users who already have an existing, valid profile.

The login webview already supports presenting existing profiles - the task here would be to wire up retrieval.