feat: support multiple error headers (#3852)

This change takes the first header value from x-amzn-errortype
if multiple headers are provided.

An example curl output for a response that this works for:
```
HTTP/2 401
date: Tue, 09 Aug 2022 21:24:20 GMT
content-type: application/json
content-length: 92
x-amzn-requestid: xxxx
access-control-allow-origin: *
x-amzn-errortype: UnauthorizedError
x-amzn-errortype: UnauthorizedException
x-amz-apigw-id: xxxx
x-amzn-trace-id: Root=xxxx
```
In this instance this code will select the first header value.

See related - smithy-lang/smithy#1170

codegen/smithy-aws-typescript-codegen/src/main/resources/software/amazon/smithy/aws/typescript/codegen/load-json-error-code-stub.ts

@@ -9,6 +9,9 @@ const loadRestJsonErrorCode = (output: __HttpResponse, data: any): string | unde
     if (typeof cleanValue === "number") {
       cleanValue = cleanValue.toString();
     }
+    if (cleanValue.indexOf(",") >= 0) {
+      cleanValue = cleanValue.split(",")[0];
+    }
     if (cleanValue.indexOf(":") >= 0) {
       cleanValue = cleanValue.split(":")[0];
     }