feat: support routing GitHub API calls to custom proxy API (#1017)

GitHub currently has some limitations w.r.t permissions management on
runner groups as they all require org admin, however at our company
we're using runner groups to serve different internal teams (with
different permissions), thus we needed to deploy a custom proxy API with
our internal authentication to provide who has access to certain APIs
depending on the repository/runner group on a given org/enterprise

This change just allows to optionally send the GitHub API calls to an alternate custom
proxy URL instead of cloud github (github.com) or an enterprise URL with
basic authentication

Co-authored-by: Yusuke Kuoka <ykuoka@gmail.com>

charts/actions-runner-controller/values.yaml

@@ -21,6 +21,11 @@ enableLeaderElection: true
 # The URL of your GitHub Enterprise server, if you're using one.
 #githubEnterpriseServerURL: https://github.example.com
 
+# Override GitHub URLs in case of using proxy APIs
+#githubURL: ""
+#githubUploadURL: ""
+#runnerGithubURL: ""
+
 # Only 1 authentication method can be deployed at a time
 # Uncomment the configuration you are applying and fill in the details
 #
@@ -41,6 +46,9 @@ authSecret:
   #github_app_private_key: |
   ### GitHub PAT Configuration
   #github_token: ""
+  ### Basic auth for github API proxy
+  #github_basicauth_username: ""
+  #github_basicauth_password: ""
 
 dockerRegistryMirror: ""
 image: