List Variable Sets associated with workspace

[sebasslash]

Description

These changes build off of #520, namely I'm making a slight modification on which interface this method belongs to.

In the original PR, the method to list variable sets for a workspace was VariableSets.ListForWorkspaces() and I've changed it to: Workspaces.ListVariableSets(). Given we typically have children relationships exposed as part of the parent interface, i.e Workspaces.ListSomeChildren(), I felt this API was more natural in describing the one-to-many relationship.

It is worth noting that this endpoint is documented under our Variable Set API, however the path is /workspaces/:workspace_id/varsets so I felt like this is ultimately part of the Workspaces API.

I've also added the applyVariableSetToWorkspace() test helper, which was missing in the original PR.

Besides this, I've kept the original implementation from @tstapler (Great work if you see this!)

Testing plan

go test -v ./... -run TestWorkspacesListVariableSets -tags=integration

External links

• Original PR

[brandonc]

I think my preference is to attach this to the VariableSets interface. Here is my rationale: Nearly everything belongs to a workspace, but the non-core interface we offer in go-tfe seems to scoped to workspace settings like tags, ssh keys, and remote state consumers.

Other things associated to workspaces (config versions, state versions, notifications, variables, run tasks) don't extend the workspace interface to add support.

[sebasslash]

That's a fair point. I'll close this PR and rebase on the original.