Make spacectl authentication easier

Currently it seems like the 3 options to authenticate to spacectl are either short-lived forms of authentication or not accessible to non-admins:

• Login with browser gives a temporary API token that expires after 10 hours, and then I have to re-enter my Spacelift URL in the CLI every day. This is not sustainable for a daily workflow.

• Github PAT seems easier if your IdP is Github, but for me, it signs out my browser session whenever I use spacectl, and re-authing in my browser signs out the CLI. This is again not sustainable for a daily workflow.

• API keys remove the re-auth problem but are completely separate from my user identity, and it’s not clear that non-admins in Spacelift can create them. This also creates long-lived API credentials, which we’d prefer to avoid.

I would be fine with opening my browser to authenticate every day if I didn’t have to enter the Spacelift URL continually.