Add branch attribute to vcs_repo block of tfe_workspace data source

[kgns]

Description

Currently we cannot get the VCS Repo Branch attribute value using tfe_workspace data source as opposed to being able to specify it using the resource version. This PR adds branch attribute to the data source and fixes this duality.

Output from acceptance tests

Full test timed out after 15m, so instead of increasing the timeout and trying again, I just ran individual test named TestAccTFEWorkspaceDataSource since the changes were only in this data source. However, vcs_repo block is not being tested in any tests, so this run does not mean anything.

$ TESTARGS="-run TestAccTFEWorkspaceDataSource" make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccTFEWorkspaceDataSource -timeout 150m
?       github.com/hashicorp/terraform-provider-tfe     [no test files]
=== RUN   TestAccTFEWorkspaceDataSource_basic
--- PASS: TestAccTFEWorkspaceDataSource_basic (9.90s)
PASS
ok      github.com/hashicorp/terraform-provider-tfe/tfe 9.907s
?       github.com/hashicorp/terraform-provider-tfe/version     [no test files]

[hashicorp-cla]

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