Add method to list variable sets by workspace

[tstapler]

Description

The Variable Sets api supports listing VariableSets using a WorkspaceID. This is useful if you only want to select the VariableSets used by an API.

Testing plan

1. Create a Workspace and add a Variable set to it.
2. Call ListForWorkspace using the workspaceID.

External links

https://www.terraform.io/enterprise/api-docs/variable-sets#list-variable-set

[hashicorp-cla]

All committers have signed the CLA.

[sebasslash]

Your changes have been merged in #552. Thank you for your contribution!