Validate tag_names for lowercase and other api validation rules

[brandonc]

Today, we print a warning if you use upcase letters in your tfe_workspace tag_names. Trouble is, you don't see these warnings until after you've already created the offending tag name and only if you have TF_LOG level set to warn.

The API will accept uppercase tag names, but will silently downcase them, which introduces a problem for uppercase config tag names, because from that time on, the case change is detected as drift. This proactive enforcement will protect users from unexpected drift on the next apply:

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # tfe_workspace.deleteme will be created
  + resource "tfe_workspace" "deleteme" {
      + allow_destroy_plan            = true
      + auto_apply                    = false
      + execution_mode                = "remote"
      + file_triggers_enabled         = true
      + force_delete                  = false
      + global_remote_state           = (known after apply)
      + id                            = (known after apply)
      + name                          = "deleteme"
      + operations                    = (known after apply)
      + organization                  = "hashicorp"
      + project_id                    = (known after apply)
      + queue_all_runs                = true
      + remote_state_consumer_ids     = (known after apply)
      + speculative_enabled           = true
      + structured_run_output_enabled = true
      + tag_names                     = [
          + "Hello-World",
        ]
      + terraform_version             = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

tfe_workspace.deleteme: Creating...
tfe_workspace.deleteme: Creation complete after 1s [id=ws-WJipFZUjydzfwvPw]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.
~/hashicorp/example-tfe-workspace
$ terraform apply
tfe_workspace.deleteme: Refreshing state... [id=ws-WJipFZUjydzfwvPw]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # tfe_workspace.deleteme will be updated in-place
  ~ resource "tfe_workspace" "deleteme" {
        id                            = "ws-WJipFZUjydzfwvPw"
        name                          = "deleteme"
      ~ tag_names                     = [
          + "Hello-World",
          - "hello-world",
        ]
        # (17 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value:

Closes #529

Description

Describe why you're making this change.

Remember to:

• Update the Change Log

• Update the Documentation

Testing plan

1. Create workspace config. Try adding invalid tags (containing uppercase characters for instance)
2. See tests to show examples of invalid tags

Example config:

resource "tfe_workspace" "deleteme" {
  name = "deleteme"
  organization = "hashicorp"
  tag_names = ["Hello"]
}

[Uk1288]

LGTM! 🎉