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/admin-rpc_test.go

@@ -40,11 +40,8 @@ import (
 ///////////////////////////////////////////////////////////////////////////////
 
 func testAdminCmdRunnerSignalService(t *testing.T, client adminCmdRunner) {
-	tmpGlobalServiceSignalCh := globalServiceSignalCh
+	defer func(sigChan chan serviceSignal) { globalServiceSignalCh = sigChan }(globalServiceSignalCh)
 	globalServiceSignalCh = make(chan serviceSignal, 10)
-	defer func() {
-		globalServiceSignalCh = tmpGlobalServiceSignalCh
-	}()
 
 	testCases := []struct {
 		signal    serviceSignal