add supports for cmd: terraform add

[magodo]

Fix #208

Though the official release of the v1.1.0 is not shipped yet, while the alpha version is available. This means general users can use the terraform add feature already via the alpha CLI tool. As the tfinstall is able to install the alpha release, it means the tfexec users should also be able to use the alpha features (i.e. the add).

For the reason above, I've added a Latest_v1_1_alpha version for the test.

[radeksimko]

Thank you for the PR @magodo

I wouldn't say it's necessarily a rule that any tfinstall-installable version has to be supported in tfexec but as long as it's correctly version-gated then I think it's fine! 😄

The add command may still undergo some changes before getting out of alpha, but it's also equally likely that it will just land as is. With that in mind I also proposed a minor change in the comment, so that folks are more aware of potential BCs. That said they probably will be aware anyway as they have to opt into installing alpha version 🤷🏻‍♂️

The PR looks pretty good aside from my in-line comment about the out flag and the signature. Once that is addressed I'd be happy to merge it.

[magodo]

@radeksimko Thank you for the quick feedback! I've resolved the comments above, please take another look :)

[radeksimko]

Looking good, I'm happy to merge once that last nitpick is addressed 😃