Add AddComment on Run struct

[alex-ikse]

Description

Add the ability to use the endpoint POST /runs/:run_id/comments to add comments on a run.

Testing plan

runComment := tfe.RunCommentOptions{
    Body: tfe.String("Test from fork of go-tfe"),
}
err = client.Runs.AddComment(ctx, run.ID, runComment)
if err != nil {
    log.Fatal(err)
}

External links

This endpoint is not documented right now.

Output from tests (HashiCorp employees only)

$ TFE_ADDRESS="https://example" TFE_TOKEN="example" TF_ACC="1" go test ./... -v -tags=integration -run TestFunctionsAffectedByChange

...

[alex-ikse]

Replaced by #355