Fix ignored json meta tags for run variables in RunCreateOptions

[Uk1288]

Description:

**NOTE: TheVariables field in Run has been changed from type []*RunVariable to []*RunVariableAttr **

Ref PR: https://github.com/hashicorp/go-tfe/pull/204

Run variables in RunCreateOptions are not marshalled in line with the API (https://www.terraform.io/cloud-docs/api-docs/run#request-body). The variables are currently marshalled as:

"variables": [
        {
          "Key": "aKey",
          "Value": "\"aValue\""
        }
]

instead of

"variables": [
        {
          "key": "aKey",
          "value": "\"aValue\""
        }
]

This PR adds changes that marshals and un-marshals the run variables correctly.

Testing plan

1. Create a run with options

	newRun, err := client.Runs.Create(context.Background(), tfe.RunCreateOptions{
		Workspace: &tfe.Workspace{
			ID: "ws-ID",
		},
		Variables: []*tfe.RunVariableOption{
			{
				Key:   "name",
				Value: "\"aName\"",
			},
		},
	})

1. Run should be created with correct variable key, name attributes,
2. Run value in go client should have Key, Value fields populated as well.

External links

• API documentation
• Issue

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://example" TFE_TOKEN="example" TF_ACC="1" go test ./... -v -tags=integration -run TestRunCreateOptions_Marshal

=== RUN   TestRunCreateOptions_Marshal
--- PASS: TestRunCreateOptions_Marshal (1.47s)
PASS
ok 

[brandonc]

@annawinkler It's not a breaking change because the behavior doesn't match the documentation. (It capitalizes the keys in the request, which technically works but it's not expected) So this is just a bug fix.

[brandonc]

@annawinkler I see now that you mean modifying the actual type. Let me think about it. It would probably be less impactful to add a new output type and redefine the existing input type.

[laurenolivia]

Hmm looks like StateVersion is flaking on your PR 🤔 But I checked out this branch and you have the latest changes.

[laurenolivia]

Testing plan works as expected! ✅

[github-actions[bot]]

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.