Add support for session policies in STS APIs (#7747)

This PR adds support for adding session policies
for further restrictions on STS credentials, useful
in situations when applications want to generate
creds for multiple interested parties with different
set of policy restrictions.

This session policy is not mandatory, but optional.

Fixes #7732

pkg/iam/policy/constants.go

@@ -20,6 +20,12 @@ import (
 	"github.com/minio/minio/pkg/policy"
 )
 
+// Policy claim constants
+const (
+	PolicyName        = "policy"
+	SessionPolicyName = "sessionPolicy"
+)
+
 // ReadWrite - provides full access to all buckets and all objects
 var ReadWrite = Policy{
 	Version: DefaultVersion,