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.

pkg/madmin/API.md

@@ -40,6 +40,8 @@ func main() {
 |:----------------------------|:----------------------------|:--------------------------------------|:--------------------------|:------------------------------------|
 | [`ServiceStatus`](#ServiceStatus) | [`ServerInfo`](#ServerInfo) | [`Heal`](#Heal) | [`GetConfig`](#GetConfig) | [`SetCredentials`](#SetCredentials) |
 | [`ServiceSendAction`](#ServiceSendAction) | | | [`SetConfig`](#SetConfig) | |
+| | |            | [`GetConfigKeys`](#GetConfigKeys) |                                     |
+| | |            | [`SetConfigKeys`](#SetConfigKeys) |                                     |
 
 
 ## 1. Constructor
@@ -326,6 +328,47 @@ __Example__
     log.Println("SetConfig: ", string(buf.Bytes()))
 ```
 
+<a name="GetConfigKeys"></a>
+### GetConfigKeys(keys []string) ([]byte, error)
+Get a json document which contains a set of keys and their values from config.json.
+
+__Example__
+
+``` go
+    configBytes, err := madmClnt.GetConfigKeys([]string{"version", "notify.amqp.1"})
+    if err != nil {
+        log.Fatalf("failed due to: %v", err)
+    }
+
+    // Pretty-print config received as json.
+    var buf bytes.Buffer
+    err = json.Indent(buf, configBytes, "", "\t")
+    if err != nil {
+        log.Fatalf("failed due to: %v", err)
+    }
+
+    log.Println("config received successfully: ", string(buf.Bytes()))
+```
+
+
+<a name="SetConfigKeys"></a>
+### SetConfigKeys(params map[string]string) error
+Set a set of keys and values for Minio server or distributed setup and restart the Minio
+server for the new configuration changes to take effect.
+
+__Example__
+
+``` go
+    err := madmClnt.SetConfigKeys(map[string]string{"notify.webhook.1": "{\"enable\": true, \"endpoint\": \"http://example.com/api\"}"})
+    if err != nil {
+        log.Fatalf("failed due to: %v", err)
+    }
+
+    log.Println("New configuration successfully set")
+```
+
+
+
 ## 8. Misc operations
 
 <a name="SetCredentials"></a>