feat(lambda): allow running a build file (#30196)

### Issue # (if applicable)

Closes #18470

### Reason for this change

This allows customers to execute an arbitrary build script as part of cdk synth, which will enable customer to use esbuild plugins. The rationale for this decision is given the issue that is linked above.

### Description of changes

1. Expose the code field on the `aws-lambda-nodejs` construct, so that customers can specify code in ways other than bundling, which was the default and abstracted away from customers before this change.
2. Add a new static method on Code, namely `Code.fromCustomCommand`. This method takes in the commands to run an arbitrary script during cdk synthesis that the customer provides. The customer also provides the location of the output from the buildscript. Then this output is supplied to a lambda function.

### Description of how you validated changes

manual testing (involving inspecting output in the AWS Lambda console and invoking the function), integration tests, and full unit test coverage of new changes.

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk-testing/framework-integ/test/aws-lambda-nodejs/test/integ-handlers/custom_command_input/build.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -euo pipefail
+
+directoryOfThisScript=$(cd $(dirname $0) && pwd)
+echo "directoryOfThisScript: $directoryOfThisScript"
+cp "$directoryOfThisScript/custom_command_handler.js" "$directoryOfThisScript/../custom_command_output/mylambdafile.js"

packages/@aws-cdk-testing/framework-integ/test/aws-lambda-nodejs/test/integ-handlers/custom_command_input/custom_command_handler.ts

@@ -0,0 +1,3 @@
+export async function handler() {
+  return 'NICE';
+}

packages/@aws-cdk-testing/framework-integ/test/aws-lambda-nodejs/test/integ-handlers/custom_command_output/README.md

@@ -0,0 +1 @@
+Code gets placed in here from the integ.specifycode.ts test