Modeling `User.Permissions` as struct instead of `map[string]bool`

[Not-Dhananjay-Mishra]

Similar to #3934
According to the List Collaborators Docs, the permissions object has a fixed, well-defined structure:

{
  "permissions": {
    "type": "object",
    "properties": {
      "pull": {
        "type": "boolean"
      },
      "triage": {
        "type": "boolean"
      },
      "push": {
        "type": "boolean"
      },
      "maintain": {
        "type": "boolean"
      },
      "admin": {
        "type": "boolean"
      }
    }
  }
}

Currently it is typed as map[string]bool (It was added in 2015 so I guess it is safe to update):

go-github/github/users.go

Lines 69 to 73 in 1c42933

	// Permissions and RoleName identify the permissions and role that a user has on a given
	// repository. These are only populated when calling Repositories.ListCollaborators.
	Permissions map[string]bool `json:"permissions,omitempty"`
	RoleName *string `json:"role_name,omitempty"`

we can use RepositoryPermissions struct introduced in #3936

[Not-Dhananjay-Mishra]

@gmlewis can we make this issue open for new contributor.

[gmlewis]

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.
Also feel free to perform a code review on any PR that has the label "NeedsReview" - we appreciate those contributions too!
We have a REVIEWERS file that you could add your username to if you are willing to be contacted when reviews are needed.

Please check out our CONTRIBUTING.md guide to get started. (In particular, please remember to go generate ./... and don't use force-push to your PRs.)

Thank you!

[JosueT1212]

I would like to work on this one !

[gmlewis]

I would like to work on this one !

Thank you, @JosueT1212! It's yours.