Use enums for the action field in GitHub Webhooks

[prnvbn]

Issue Description:

Many GitHub webhooks contain the action field, which is enumerated. For example, in the GitHub documentation for webhook payloads and events, the check_run event includes an action property that can take on values such as "completed", "created", "requested_action", and "rerequested". However, in the library, these fields are currently represented as free-form strings. In my opinion, making these enums would be more user-friendly.

Proposed Solution

A rudimentary example of how this can be done is as follows (go play ground link)

package main

import (
    "encoding/json"
    "fmt"
)

type ChecRunAction string

const (
    CheckRunCreated         ChecRunAction = "created"
    CheckRunRequested       ChecRunAction = "requested"
    CheckRunRerequested     ChecRunAction = "rerequested"
    CheckRunRequestedAction ChecRunAction = "requested_action"
)

func main() {
    jsonStr := `{"hi": "created"}`

    var v struct {
        A ChecRunAction `json:"hi"`
    }
    json.Unmarshal([]byte(jsonStr), &v)
    fmt.Println(v)
}

[prnvbn]

If you agree, I can make this change

[gmlewis]

That seems safe to me.

[prnvbn]

nice, will send a PR your way!

[prnvbn]

@gmlewis, please could you review the PR: https://github.com/google/go-github/pull/3136?