Add `Owner` to `EditChange` struct for repository transfer webhook event payloads

[ecrupper]

When a repository has a transfer of ownership, it will send a repository event webhook. The structure ends up looking like this:

{
  "action": "transferred",
  "changes": {
    "owner": {
      "from": {
        "user": {
          "login": "octocat",
          ... etc

OR

{
  "action": "transferred",
  "changes": {
    "owner": {
      "from": {
        "organization": {
          "login": "github",
          ... etc

I'd like to add the missing Owner field to the EditChange struct. I was thinking it would look something like this:

type EditChange struct {
	Title *EditTitle `json:"title,omitempty"`
	Body  *EditBody  `json:"body,omitempty"`
	Base  *EditBase  `json:"base,omitempty"`
	Repo  *EditRepo  `json:"repository,omitempty"`
	Owner *EditOwner `json:"owner,omitempty"`
}

type EditOwner struct {
	OwnerInfo *OwnerInfo `json:"from,omitempty"`
}

type OwnerInfo struct {
	Actor *User `json:"user,omitempty"`
	Org   *User `json:"organization,omitempty"`
}

Unfortunately, due to the structure of the payload, OwnerInfo will have to account for those two different options, even though they both refer to owners.

[gmlewis]

Thank you, @ecrupper - that sounds reasonable.