AppSync: JS Resolvers do not support code bundled in custom bundling environment

### Describe the bug

`AppSync` now supports `JavaScript` resolvers which in `CDK` are can be populated trough the `code` prop which can either be `fromInline` or `fromAsset`.

The problem is that it seems `AppSync` can only take `.js` and will throw a `The code contains one or more errors.` exception when you use a custom bundling environment, since this emits a `.zip`.

### Expected Behavior

`AppSync` digests single file archives generated when using a custom `Docker` environment to bundle `JavaScript` `AppSync` resolvers.

### Current Behavior

`AppSync` throws a 'The code contains one or more errors.' exception when using a custom `Docker` environment to bundle `JavaScript` `AppSync` resolvers.

### Reproduction Steps

```ts
new Resolver(this, "TestResolver", {
api: new GraphqlApi(this, "TestApi", {
name: "TestApi",
schema: SchemaFile.fromAsset(path.join(__dirname, "./schema.graphql")),
}),
typeName: "Query",
fieldName: "test",
runtime: FunctionRuntime.JS_1_0_0,
code: Code.fromAsset(path.join(__dirname, "./"), {
bundling: {
image: DockerImage.fromBuild(path.join(__dirname, "./")),
command: [
"esbuild",
"/asset-input/resolver.ts",
"--target=node18",
"--outfile=/asset-output/index.js",
],
},
}),
});
```

### Possible Solution

I see two possible solution for this problem:

1. Allow single file outputs from bundling with custom environment rather then only archives
2. Somehow make AppSync phrase the archive edited from a custom environment bundle

### Additional Information/Context

_No response_

### CDK CLI Version

2.67.0

### Framework Version

_No response_

### Node.js Version

18

### OS

aws/codebuild/amazonlinux2-x86_64-standard:4.0

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start by reproducing the issue with the TypeScript Resolver and Code.fromAsset example, using the custom Docker bundling command that writes index.js to /asset-output. Trace how the bundled asset is passed to the AppSync JavaScript resolver and verify whether single-file archives are accepted. Done means the reproduced resolver deploys successfully without the code validation error.

Written by the indexing model from the issue text.