Add Variables to the Workspace relations

[arybolovlev]

Description

This PR adds Variables to the Workspace relations.

$ curl -s \
  --header "Authorization: Bearer $TFE_TOKEN" \
  --header "Content-Type: application/vnd.api+json" \
  --request GET \
"https://app.terraform.io/api/v2/workspaces/ws-XXX" | jq

    "relationships": {
      ...
      "vars": {
        "data": [
          {
            "id": "var-xh8w2W4bZJ3X4HH9",
            "type": "vars"
          },
          {
            "id": "var-PStyaxTNYWszMBoy",
            "type": "vars"
          }
        ]
      }
      ...
    }

Testing plan

N/A.

External links

Relations are purely documented in the API doc, however, vars are mentioned there one.

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

$ TFE_ADDRESS="https://app.terraform.io/" TFE_TOKEN="X.atlasv1.X" go test ./... -v -run TestVariablesCreate$

PASS
ok  	github.com/hashicorp/go-tfe	5.293s

[Maed223]

Code looks good 👍– only thing missing is the Changelog entry.

[arybolovlev]

Code looks good 👍– only thing missing is the Changelog entry.

Added a changelog. Thank you!

[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.