config: Add api requests max & deadline configs (#9273)

Add two new configuration entries, api.requests-max and
api.requests-deadline which have the same role of
MINIO_API_REQUESTS_MAX and MINIO_API_REQUESTS_DEADLINE.

cmd/config/config.go

@@ -71,6 +71,7 @@ const (
 	RegionSubSys         = "region"
 	EtcdSubSys           = "etcd"
 	StorageClassSubSys   = "storage_class"
+	APISubSys            = "api"
 	CompressionSubSys    = "compression"
 	KmsVaultSubSys       = "kms_vault"
 	KmsKesSubSys         = "kms_kes"
@@ -102,6 +103,7 @@ var SubSystems = set.CreateStringSet([]string{
 	RegionSubSys,
 	EtcdSubSys,
 	CacheSubSys,
+	APISubSys,
 	StorageClassSubSys,
 	CompressionSubSys,
 	KmsVaultSubSys,
@@ -129,6 +131,7 @@ var SubSystemsSingleTargets = set.CreateStringSet([]string{
 	RegionSubSys,
 	EtcdSubSys,
 	CacheSubSys,
+	APISubSys,
 	StorageClassSubSys,
 	CompressionSubSys,
 	KmsVaultSubSys,