Implement long poll for interactive login

[juanfont]

When using interactive login (i.e., the web + CLI or OIDC) for client registration, clients expect the control server to enter in a long poll until the client gets registered. This is shown in the mock control server tailscale has for testing https://github.com/tailscale/tailscale/blob/v1.28.0/tstest/integration/testcontrol/testcontrol.go#L451

If this long polling is not followed (or, as we are doing, an immediate response is given), the client will immediately retry... ad infinitum - hammering headscale with requests.

This is what is happening in #603 and #706.

The long-term solution is to implement some basic long poll here, that holds the request until the registration occurs (or the client closed the connection).

[juanfont]

I have included a temporary solution in e1e3feb

(should a bit help in #603 and #706)

[github-actions]

This issue is stale because it has been open for 180 days with no activity.

[github-actions]

This issue was closed because it has been inactive for 14 days since being marked as stale.