Adding missing Category to VariableUpdateOptions

[jtyr]

Description

This PR is adding missing Categoiry attribute to the VariableUpdateOptions method. Without this change, nobody can change the category of a Workspace variable and the only way how to remediate this is to delete the variable and create it again.

Testing plan

1. Find a way how to change the category of a Workspace variable (there is none)

External links

The official API documentation shows that it should be possible to change the category attribute.

Output from tests (HashiCorp employees only)

$ docker run -it --rm -e TFE_ADDRESS='https://app.terraform.io' -e TFE_TOKEN="$TFE_TOKEN" -e ENABLE_TFE='1' -e TEST_RESULTS_DIR='/tmp/test_results' -v $PWD:/tmp/code --workdir /tmp/code docker.mirror.hashicorp.services/cimg/go:1.17
circleci@2abe77f7293c:/tmp/code$ mkdir -p $TEST_RESULTS_DIR
circleci@2abe77f7293c:/tmp/code$ export GO111MODULE=on
circleci@2abe77f7293c:/tmp/code$ gotestsum --format short-verbose --junitfile $TEST_RESULTS_DIR/tests.xml -- -timeout=40m -tags=integration -run TestVariablesUpdate
go: downloading github.com/google/go-querystring v1.1.0
go: downloading github.com/hashicorp/go-cleanhttp v0.5.2
go: downloading github.com/hashicorp/go-retryablehttp v0.7.0
go: downloading github.com/hashicorp/go-slug v0.8.0
go: downloading github.com/hashicorp/jsonapi v0.0.0-20210826224640-ee7dae0fb22d
go: downloading golang.org/x/time v0.0.0-20190308202827-9d24e82272b4
go: downloading github.com/hashicorp/go-uuid v1.0.2
go: downloading github.com/stretchr/testify v1.7.0
go: downloading gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c
go: downloading github.com/pmezard/go-difflib v1.0.0
go: downloading github.com/davecgh/go-spew v1.1.1
PASS TestVariablesUpdate/with_valid_options (0.33s)
PASS TestVariablesUpdate/when_updating_a_subset_of_values (0.70s)
PASS TestVariablesUpdate/with_sensitive_set (0.31s)
PASS TestVariablesUpdate/with_category_set (0.61s)
PASS TestVariablesUpdate/without_any_changes (1.85s)
PASS TestVariablesUpdate/with_invalid_variable_ID (0.00s)
PASS TestVariablesUpdate/with_invalid_variable_ID#01 (0.00s)
PASS TestVariablesUpdate (5.58s)
PASS .

DONE 8 tests in 15.338s

[sebasslash]

Changes look good 👍

[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.