feat: Added back TF_WORKSPACE support

[vfoucault]

Since I don't know why TF_WORKSPACE env var has been explicitly removed in #75, I propose to add it back to resolve #247

When you want to perform some terraform operation in parallel in different workspaces, there are two option, either you copy the terraform code into another directory or use the TF_WORKSPACE variable.

I've added a WorkSpaceOption for each and every related command since copying and duplicating code is not an option

usage:

var options = []tfexec.ApplyOption{
		tfexec.WorkSpace("some_workspace"),
	}

[vfoucault]

@kmoe @radeksimko I know guys that you might be busy on other tasks, but could you by any chance give some time to review this PR ? Many thanks !

[hashicorp-cla]

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