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>

github/github.go

@@ -23,6 +23,11 @@ type Config struct {
 	AppInstallationID int64  `split_words:"true"`
 	AppPrivateKey     string `split_words:"true"`
 	Token             string
+	URL               string `split_words:"true"`
+	UploadURL         string `split_words:"true"`
+	BasicauthUsername string `split_words:"true"`
+	BasicauthPassword string `split_words:"true"`
+	RunnerGitHubURL   string `split_words:"true"`
 }
 
 // Client wraps GitHub client with some additional
@@ -34,10 +39,23 @@ type Client struct {
 	GithubBaseURL string
 }
 
+type BasicAuthTransport struct {
+	Username string
+	Password string
+}
+
+func (p BasicAuthTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	req.SetBasicAuth(p.Username, p.Password)
+	req.Header.Set("User-Agent", "actions-runner-controller")
+	return http.DefaultTransport.RoundTrip(req)
+}
+
 // NewClient creates a Github Client
 func (c *Config) NewClient() (*Client, error) {
 	var transport http.RoundTripper
-	if len(c.Token) > 0 {
+	if len(c.BasicauthUsername) > 0 && len(c.BasicauthPassword) > 0 {
+		transport = BasicAuthTransport{Username: c.BasicauthUsername, Password: c.BasicauthPassword}
+	} else if len(c.Token) > 0 {
 		transport = oauth2.NewClient(context.Background(), oauth2.StaticTokenSource(&oauth2.Token{AccessToken: c.Token})).Transport
 	} else {
 		var tr *ghinstallation.Transport
@@ -63,6 +81,7 @@ func (c *Config) NewClient() (*Client, error) {
 		}
 		transport = tr
 	}
+
 	transport = metrics.Transport{Transport: transport}
 	httpClient := &http.Client{Transport: transport}
 
@@ -78,6 +97,35 @@ func (c *Config) NewClient() (*Client, error) {
 	} else {
 		client = github.NewClient(httpClient)
 		githubBaseURL = "https://github.com/"
+
+		if len(c.URL) > 0 {
+			baseUrl, err := url.Parse(c.URL)
+			if err != nil {
+				return nil, fmt.Errorf("github client creation failed: %v", err)
+			}
+			if !strings.HasSuffix(baseUrl.Path, "/") {
+				baseUrl.Path += "/"
+			}
+			client.BaseURL = baseUrl
+		}
+
+		if len(c.UploadURL) > 0 {
+			uploadUrl, err := url.Parse(c.UploadURL)
+			if err != nil {
+				return nil, fmt.Errorf("github client creation failed: %v", err)
+			}
+			if !strings.HasSuffix(uploadUrl.Path, "/") {
+				uploadUrl.Path += "/"
+			}
+			client.UploadURL = uploadUrl
+		}
+
+		if len(c.RunnerGitHubURL) > 0 {
+			githubBaseURL = c.RunnerGitHubURL
+			if !strings.HasSuffix(githubBaseURL, "/") {
+				githubBaseURL += "/"
+			}
+		}
 	}
 
 	return &Client{