Add admin get/set config keys API (#6113)

This PR adds two new admin APIs in Minio server and madmin package:
- GetConfigKeys(keys []string) ([]byte, error)
- SetConfigKeys(params map[string]string) (err error)

A key is a path in Minio configuration file, (e.g. notify.webhook.1)

The user will always send a string value when setting it in the config file,
the API will know how to convert the value to the appropriate type. The user
is also able to set a raw json.

Before setting a new config, Minio will validate all fields and try to connect
to notification targets if available.

cmd/config-current_test.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"context"
 	"os"
 	"path"
 	"testing"
@@ -51,7 +52,7 @@ func TestServerConfig(t *testing.T) {
 		t.Errorf("Expecting version %s found %s", globalServerConfig.GetVersion(), serverConfigVersion)
 	}
 
-	if err := saveServerConfig(objLayer, globalServerConfig); err != nil {
+	if err := saveServerConfig(context.Background(), objLayer, globalServerConfig); err != nil {
 		t.Fatalf("Unable to save updated config file %s", err)
 	}