Implement AssumeRole API for Minio users (#7267)

For actual API reference read here

https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html

Documentation is added and updated as well at docs/sts/assume-role.md

Fixes #6381

cmd/iam.go

@@ -28,7 +28,7 @@ import (
 	"github.com/minio/minio-go/pkg/set"
 	"github.com/minio/minio/cmd/logger"
 	"github.com/minio/minio/pkg/auth"
-	"github.com/minio/minio/pkg/iam/policy"
+	iampolicy "github.com/minio/minio/pkg/iam/policy"
 	"github.com/minio/minio/pkg/madmin"
 )
 
@@ -326,6 +326,27 @@ func (sys *IAMSys) SetTempUser(accessKey string, cred auth.Credentials, policyNa
 	return nil
 }
 
+// GetUserPolicy - returns canned policy name associated with a user.
+func (sys *IAMSys) GetUserPolicy(accessKey string) (policyName string, err error) {
+	objectAPI := newObjectLayerFn()
+	if objectAPI == nil {
+		return "", errServerNotInitialized
+	}
+
+	sys.RLock()
+	defer sys.RUnlock()
+
+	if _, ok := sys.iamUsersMap[accessKey]; !ok {
+		return "", errNoSuchUser
+	}
+
+	if _, ok := sys.iamPolicyMap[accessKey]; !ok {
+		return "", errNoSuchUser
+	}
+
+	return sys.iamPolicyMap[accessKey], nil
+}
+
 // ListUsers - list all users.
 func (sys *IAMSys) ListUsers() (map[string]madmin.UserInfo, error) {
 	objectAPI := newObjectLayerFn()