Add IsCloud() and IsEnterprise() helper methods to client

[sebasslash]

Description

These are helper methods that will enable external tools using a go-tfe client to make TFC or TFE-specific decisions. Thus far we haven't had an authoritative, simple way to determine if an instance is TFC or TFE. TFC and future TFE releases will start including a header TFP-AppName which will let us know if the given instance is TFC or TFE. For most users however, they will know beforehand what kind of instance their client is interacting with.

[nfagerlund]

I don't have a TFE instance around to trivially test this against, but it works against cloud right now, and the code looks fine. (Though, I can't seem to find where in the TFC code we actually add the header. 😅)

[sebasslash]

This header won't be available until the next TFE release and I don't have a way of spinning up a TFE box, I'll just wait for nightly CI. You can find the header in the api/v2 base controller, search for app_name. This change was mostly for determining if an instance was TFC, since not every TFE release includes the TFE version header or the AppName header.

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.