Adding missing Category for Workspace Variable

[jtyr]

This is a draft PR that is adding missing functionality to change variable category once the variable was already created in TFC. It's a draft because there is missing functionality to set the variable category in the upstream TFE SDK. I have created PR to add the functionality to the SDK so as soon as that's merged, I will amend this PR with updated upstream SDK.

[jtyr]

I have upgraded the go-tfe dependency to the latest version to get the Category support available. As the go-tfe interface changed a little bit, I had to modify some other bits of the code to reflect that.