RFC 113: Implement `PermsFetcher` interface for GitHub authz provider

Created by: unknwon

Essentially, add two methods to GitHub authz provider: enterprise/cmd/frontend/internal/authz/github

https://github.com/sourcegraph/sourcegraph/blob/21ebcf76705dff32b5c6573de65e12ab9c5a112d/enterprise/cmd/repo-updater/authz/perms_syncer.go#L45-L62

An example implementation for GitLab can be found at https://github.com/sourcegraph/sourcegraph/blob/master/enterprise/cmd/frontend/internal/authz/gitlab/sudo.go

Things to note

1. Return partial but valid results whenever possible.
2. When fetch in user-centric:
   1. Use token extracted from github.GetExternalAccountData(&userAccount.ExternalAccountData).
   2. The GraphQL node ID as the extsvc.ExternalAccountID for results.
   3. The API is same as what we use for RepoPerms today.
3. When fetch in repo-centric:
   1. Use the admin token configured in external service.
   2. The GraphQL node ID as the extsvc.ExternalRepoID for results.
   3. API example (login is not required, just for debugging):

   {
     repository(name: "sourcegraph", owner: "sourcegraph") {
       id
       collaborators(affiliation: ALL) {
         edges {
           node {
             id
             login
           }
         }
         pageInfo {
           endCursor
           hasNextPage
         }
       }
     }
   }

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Let me know when you have any questions!