command: Enable terraform login command, add manual token support

[alisdair]

The token generated in the above screen capture has been invalidated 🤫

This is the first in a series of PRs implementing and improving the terraform login feature.

Changes

• Enable the stubbed-out terraform login sub-command
• Add fallback detection of the tfe.v2 API (to ensure that we're trying to authenticate against compatible Terraform Cloud/Terraform Enterprise)
• If login.v1 is supported, use whichever OAuth grant flow is available (note: none of these are supported by TFC/TFE yet)
• Otherwise, if tfe.v2 is supported, open a browser to the user's token settings page, and wait for a token to be pasted back

[mildwonkey]

It looks like this if statement is redundant, since we've already exhausted the cases if clientConfig == nil - what do you think?

[alisdair]

I think it's still needed. The previous if statement doesn't result in an early exit, and only triggers an additional service discovery step. So we could still arrive at this line with a non-nil clientConfig, if the target host supports the login.v1 service.

I don't think we can even move this block up to an else clause, because we want to print diagnostics and exit if neither service is supported. The control flow after these changes is definitely more confusing, but I can't find any way to simplify it.

[mildwonkey]

gotchya, that's really obvious in hindsight - thanks!

[ghost]

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