Calculate content-length for SSDKs

This updates SSDK generation so that content-length is properly
calculated. The existing functionality for clients couldn't be
re-used because it is designed to function as a middleware that runs
just beforee the request is sent. That makes sense for the client,
because a middleware could be modifying the body. But since the SSDK
doesn't have middleware, we just embed it directly.

Author: JordonPhillips

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/integration/HttpBindingProtocolGenerator.java

@@ -506,6 +506,8 @@ private void generateOperationResponseSerializer(
                         .forEach(serializingDocumentShapes::add);
             }
 
+            calculateContentLength(context);
+
             writer.openBlock("return new $T({", "});", responseType, () -> {
                 writer.write("headers,");
                 writer.write("body,");
@@ -519,6 +521,20 @@ private void generateOperationResponseSerializer(
         }
     }
 
+    private void calculateContentLength(GenerationContext context) {
+        TypeScriptWriter writer = context.getWriter();
+        writer.addDependency(TypeScriptDependency.AWS_SDK_UTIL_BODY_LENGTH_NODE);
+        writer.addImport("calculateBodyLength", null, "@aws-sdk/util-body-length-node");
+        writer.openBlock("if (body && Object.keys(headers).map((str) => str.toLowerCase())"
+                + ".indexOf('content-length') === -1) {", "}", () -> {
+            writer.write("const length = calculateBodyLength(body);");
+            writer.openBlock("if (length !== undefined) {", "}", () -> {
+                writer.write("headers = { ...headers, 'content-length': String(length) };");
+            });
+        });
+
+    }
+
     private void generateErrorSerializer(GenerationContext context, StructureShape error) {
         SymbolProvider symbolProvider = context.getSymbolProvider();
         Symbol symbol = symbolProvider.toSymbol(error);
@@ -846,7 +862,7 @@ private void writeResponseHeaders(
         TypeScriptWriter writer = context.getWriter();
 
         // Headers are always present either from the default document or the payload.
-        writer.openBlock("const headers: any = {", "};", () -> {
+        writer.openBlock("let headers: any = {", "};", () -> {
             writeContentTypeHeader(context, operationOrError, false);
             injectExtraHeaders.run();