Add support for Identity Management Plugin (#14913)

- Adds an STS API `AssumeRoleWithCustomToken` that can be used to 
  authenticate via the Id. Mgmt. Plugin.
- Adds a sample identity manager plugin implementation
- Add doc for plugin and STS API
- Add an example program using go SDK for AssumeRoleWithCustomToken

cmd/globals.go

@@ -41,6 +41,7 @@ import (
 	"github.com/minio/minio/internal/config/dns"
 	xldap "github.com/minio/minio/internal/config/identity/ldap"
 	"github.com/minio/minio/internal/config/identity/openid"
+	idplugin "github.com/minio/minio/internal/config/identity/plugin"
 	xtls "github.com/minio/minio/internal/config/identity/tls"
 	polplugin "github.com/minio/minio/internal/config/policy/plugin"
 	"github.com/minio/minio/internal/config/storageclass"
@@ -200,10 +201,13 @@ var (
 	globalAPIConfig = apiConfig{listQuorum: "strict"}
 
 	globalStorageClass storageclass.Config
+
 	globalLDAPConfig   xldap.Config
 	globalOpenIDConfig openid.Config
 	globalSTSTLSConfig xtls.Config
 
+	globalAuthNPlugin *idplugin.AuthNPlugin
+
 	// CA root certificates, a nil value means system certs pool will be used
 	globalRootCAs *x509.CertPool