SAM debug configurations don't allow non-JSON payloads to send to API

**Describe the bug**

When using debug configurations for an AWS SAM application using API invocation you are unable to pass a payload containing anything other than JSON data however this works just fine if you start up the API using SAM CLI and then execute a curl request to the API endpoint. It also works in live environments.

**To Reproduce**
Steps to reproduce the behavior:
1. Create the sample HelloWorld SAM application using the VSCode Toolkit
2. Add an API debug configuration like this one I've included below.
3. Try setting the path to a file that contains XML rather than JSON data.
4. See error saying it's an invalid JSON file.

**Expected behavior**

I should be able to pass a payload containing any type of data I wish, just like you can with a normal API gateway / lambda combination.

Debug configuration showing what I tried:
```json
{
"type": "aws-sam",
"request": "direct-invoke",
"name": "API someapp:SomeApp (java8)",
"invokeTarget": {
"target": "api",
"templatePath": "${workspaceFolder}/someapp/template.yaml",
"logicalId": "SomeApp"
},
"api": {
"path": "/hello",
"httpMethod": "post",
"payload": {
"path": "${workspaceFolder}/someapp/events/test.xml"
},
"headers": {
"accept": "application/json",
"content-type": "application/xml",
}

},
"lambda": {
"runtime": "java8"
}
}
```

Contents of test.xml:
```xml
greeting
```

Here's the steps showing I can accomplish what I"m trying to accomplish outside of the debug configuration but not with it:
1. `sam build --use-container`
2. `sam local start-api`
3. `curl http://localhost:3000/hello -d 'greeting'`

Trace the SAM API debug configuration handling for a payload loaded from the configured path, focusing on where the file is treated as JSON. Compare that behavior with the documented sam local start-api and curl flow. Done means an XML or other non-JSON payload can be sent through the debug configuration while existing JSON payloads continue to work.

Written by the indexing model from the issue text.