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/crypto/config.go

@@ -16,5 +16,6 @@ package crypto
 
 // KMSConfig has the KMS config for hashicorp vault
 type KMSConfig struct {
-	Vault VaultConfig `json:"vault"`
+	AutoEncryption bool        `json:"-"`
+	Vault          VaultConfig `json:"vault"`
 }