add auto-encryption feature (#6523)

This commit adds an auto-encryption feature which allows
the Minio operator to ensure that uploaded objects are
always encrypted.

This change adds the `autoEncryption` configuration option
as part of the KMS conifguration and the ENV. variable
`MINIO_SSE_AUTO_ENCRYPTION:{on,off}`.

It also updates the KMS documentation according to the
changes.

Fixes #6502

cmd/server-main.go

@@ -18,6 +18,7 @@ package cmd
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -383,6 +384,9 @@ func serverMain(ctx *cli.Context) {
 	if err = globalNotificationSys.Init(newObject); err != nil {
 		logger.LogIf(context.Background(), err)
 	}
+	if globalAutoEncryption && !newObject.IsEncryptionSupported() {
+		logger.Fatal(errors.New("Invalid KMS configuration"), "auto-encryption is enabled but server does not support encryption")
+	}
 
 	globalObjLayerMutex.Lock()
 	globalObjectAPI = newObject