Adds html_url attribute to d/tfe_workspace and r/tfe_workspace

[brandonc]

Description

Adds html_url attribute to workspace resource & data source which is the URL to the browsable terraform cloud workspace overview as reported by the API

• This PR is blocked until the next go-tfe release (1.18)

Output from acceptance tests

$ TESTARGS="-run TestAccTFEWorkspace_basic" make testacc
TF_ACC=1 TF_LOG_SDK_PROTO=OFF go test $(go list ./... |grep -v 'vendor') -v -run TestAccTFEWorkspace_basic -timeout 15m
?   	github.com/hashicorp/terraform-provider-tfe	[no test files]
=== RUN   TestAccTFEWorkspace_basic
--- PASS: TestAccTFEWorkspace_basic (38.37s)
=== RUN   TestAccTFEWorkspace_basicReadProjectId
    testing.go:186: Skipping test related to a Terraform Cloud/Enterprise beta feature. Set ENABLE_BETA=1 to run.
--- SKIP: TestAccTFEWorkspace_basicReadProjectId (0.00s)
=== RUN   TestAccTFEWorkspace_basicAssessmentsEnabled
--- PASS: TestAccTFEWorkspace_basicAssessmentsEnabled (61.81s)
PASS
ok  	github.com/hashicorp/terraform-provider-tfe/tfe	100.627s
?   	github.com/hashicorp/terraform-provider-tfe/version	[no test files]
...