Add custom policy claim name (#8764)

In certain organizations policy claim names
can be not just 'policy' but also things like
'roles', the value of this field might also
be *string* or *[]string* support this as well

In this PR we are still not supporting multiple
policies per STS account which will require a
more comprehensive change.

pkg/iam/policy/policy.go

@@ -19,6 +19,7 @@ package iampolicy
 import (
 	"encoding/json"
 	"io"
+	"strings"
 
 	"github.com/minio/minio/pkg/policy"
 )
@@ -37,6 +38,20 @@ type Args struct {
 	Claims          map[string]interface{} `json:"claims"`
 }
 
+// GetPolicies get policies
+func (a Args) GetPolicies(policyClaimName string) ([]string, bool) {
+	pname, ok := a.Claims[policyClaimName]
+	if !ok {
+		return nil, false
+	}
+	pnameStr, ok := pname.(string)
+	if ok {
+		return strings.Split(pnameStr, ","), true
+	}
+	pnameSlice, ok := pname.([]string)
+	return pnameSlice, ok
+}
+
 // Policy - iam bucket iamp.
 type Policy struct {
 	ID         policy.ID `json:"ID,omitempty"`