Add ephemeral `miren login` that doesn't persistent keys

There are many cases where it would be fine for miren login to just store a JWT and reuse it, rather than performing the persistent key dance. We should add this and it should become the default.

The persistent key mechanism is really nice for long lived logins, but it puts a lot of stress on the backend for a feature that most users don't use.

In the future, we can look to make the persistent key mechanism more robust so that it doesn't generate new key on every login.