Add WithTransport option to set TLS config (#102)

Users can provide an http.Transport instance with non-default TLS,
proxy, or timeout settings.

githubapp/client_creator.go

@@ -135,6 +135,7 @@ type clientCreator struct {
 	cacheFunc      func() httpcache.Cache
 	alwaysValidate bool
 	timeout        time.Duration
+	transport      http.RoundTripper
 }
 
 var _ ClientCreator = &clientCreator{}
@@ -178,6 +179,15 @@ func WithClientMiddleware(middleware ...ClientMiddleware) ClientOption {
 	}
 }
 
+// WithTransport sets the http.RoundTripper used to make requests. Clients can
+// provide an http.Transport instance to modify TLS, proxy, or timeout options.
+// By default, clients use http.DefaultTransport.
+func WithTransport(transport http.RoundTripper) ClientOption {
+	return func(c *clientCreator) {
+		c.transport = transport
+	}
+}
+
 func (c *clientCreator) NewAppClient() (*github.Client, error) {
 	base := c.newHTTPClient()
 	installation, transportError := newAppInstallation(c.integrationID, c.privKeyBytes, c.v3BaseURL)
@@ -282,7 +292,7 @@ func (c *clientCreator) NewTokenSourceV4Client(ts oauth2.TokenSource) (*githubv4
 
 func (c *clientCreator) newHTTPClient() *http.Client {
 	return &http.Client{
-		Transport: http.DefaultTransport,
+		Transport: c.transport,
 		Timeout:   c.timeout,
 	}
 }