s3: allow single Statement object in policy document (#8212)

* s3: allow single Statement object in policy document

Fixes #8201

* s3: add unit test for single Statement object in policy

* s3: improve error message for malformed PolicyDocument.Statement

* s3: simplify error message for malformed PolicyDocument.Statement

weed/s3api/policy_engine/engine_test.go

@@ -286,6 +286,18 @@ func TestPolicyValidation(t *testing.T) {
 			}`,
 			expectError: false,
 		},
+		{
+			name: "Valid policy with single statement object",
+			policyJSON: `{
+				"Version": "2012-10-17",
+				"Statement": {
+					"Effect": "Allow",
+					"Action": "s3:GetObject",
+					"Resource": "arn:aws:s3:::test-bucket/*"
+				}
+			}`,
+			expectError: false,
+		},
 		{
 			name: "Invalid version",
 			policyJSON: `{