Migrate config to KV data format (#8392)

- adding oauth support to MinIO browser (#8400) by @kanagaraj
- supports multi-line get/set/del for all config fields
- add support for comments, allow toggle
- add extensive validation of config before saving
- support MinIO browser to support proper claims, using STS tokens
- env support for all config parameters, legacy envs are also
  supported with all documentation now pointing to latest ENVs
- preserve accessKey/secretKey from FS mode setups
- add history support implements three APIs
  - ClearHistory
  - RestoreHistory
  - ListHistory
- add help command support for each config parameters
- all the bug fixes after migration to KV, and other bug
  fixes encountered during testing.

cmd/config/storageclass/storage-class.go

@@ -36,6 +36,11 @@ const (
 
 // Standard constats for config info storage class
 const (
+	ClassStandard = "standard"
+	ClassRRS      = "rrs"
+
+	// Env to on/off storage class settings.
+	EnvStorageClass = "MINIO_STORAGE_CLASS_STATE"
 	// Reduced redundancy storage class environment variable
 	RRSEnv = "MINIO_STORAGE_CLASS_RRS"
 	// Standard storage class environment variable
@@ -51,6 +56,16 @@ const (
 	defaultRRSParity = minParityDisks
 )
 
+// DefaultKVS - default storage class config
+var (
+	DefaultKVS = config.KVS{
+		config.State:   config.StateOff,
+		config.Comment: "This is a default StorageClass configuration, only applicable in erasure coded setups",
+		ClassStandard:  "",
+		ClassRRS:       "EC:2",
+	}
+)
+
 // StorageClass - holds storage class information
 type StorageClass struct {
 	Parity int
@@ -196,11 +211,25 @@ func (sCfg Config) GetParityForSC(sc string) (parity int) {
 }
 
 // LookupConfig - lookup storage class config and override with valid environment settings if any.
-func LookupConfig(cfg Config, drivesPerSet int) (Config, error) {
-	var err error
+func LookupConfig(kvs config.KVS, drivesPerSet int) (cfg Config, err error) {
+	cfg = Config{}
+	cfg.Standard.Parity = drivesPerSet / 2
+	cfg.RRS.Parity = defaultRRSParity
+
+	if err = config.CheckValidKeys(config.StorageClassSubSys, kvs, DefaultKVS); err != nil {
+		return cfg, err
+	}
+
+	stateBool, err := config.ParseBool(env.Get(EnvStorageClass, kvs.Get(config.State)))
+	if err != nil {
+		return cfg, err
+	}
+	if !stateBool {
+		return cfg, nil
+	}
 
 	// Check for environment variables and parse into storageClass struct
-	if ssc := env.Get(StandardEnv, cfg.Standard.String()); ssc != "" {
+	if ssc := env.Get(StandardEnv, kvs.Get(ClassStandard)); ssc != "" {
 		cfg.Standard, err = parseStorageClass(ssc)
 		if err != nil {
 			return cfg, err
@@ -210,7 +239,7 @@ func LookupConfig(cfg Config, drivesPerSet int) (Config, error) {
 		cfg.Standard.Parity = drivesPerSet / 2
 	}
 
-	if rrsc := env.Get(RRSEnv, cfg.RRS.String()); rrsc != "" {
+	if rrsc := env.Get(RRSEnv, kvs.Get(ClassRRS)); rrsc != "" {
 		cfg.RRS, err = parseStorageClass(rrsc)
 		if err != nil {
 			return cfg, err