[Feature] Support CLI login flow

[colinjc]

It should be possible to login to a dstack enterprise instance using the CLI with the users Okta credentials.

Proposal

dstack login https://my-instance launches a browser auth flow that logs into dstack, retrieves the users token, and configures the CLI to use it.

[r4victor]

Probably makes sense to have dstack login working in the OSS and the sign in options would be specific to OSS/Sky/Enterprise.