Adds support for private key path in config (#133)

* Adds support for private key path in config

okta/config.go

@@ -17,7 +17,10 @@
 package okta
 
 import (
+	"io/ioutil"
+	"log"
 	"net/http"
+	"os"
 
 	"github.com/okta/okta-sdk-golang/v2/okta/cache"
 )
@@ -180,8 +183,25 @@ func WithScopes(scopes []string) ConfigSetter {
 	}
 }
 
+// WithPrivateKey sets private key key. Can be either a path to a private key or private key itself.
 func WithPrivateKey(privateKey string) ConfigSetter {
 	return func(c *config) {
-		c.Okta.Client.PrivateKey = privateKey
+		if fileExists(privateKey) {
+			content, err := ioutil.ReadFile(privateKey)
+			if err != nil {
+				log.Fatalf("failed to read from provided private key file path: %v", err)
+			}
+			c.Okta.Client.PrivateKey = string(content)
+		} else {
+			c.Okta.Client.PrivateKey = privateKey
+		}
 	}
 }
+
+func fileExists(filename string) bool {
+	info, err := os.Stat(filename)
+	if os.IsNotExist(err) {
+		return false
+	}
+	return !info.IsDir()
+}