Bug: sam local generate-event does not escape special characters in input

### Description:

`sam local generate-event` does not escape special characters when interpolating values into the output.

Note that this was previously raised as #2848 but I think it was closed due to a misunderstanding. Regardless of what the body string *is* the output from `sam local generate-event` should still be valid JSON. The [comment indicating this behaviour was by design](https://github.com/aws/aws-sam-cli/issues/2848#issuecomment-933509224) seemed to confuse the issue as being with the body text being JSON.

In my reproduction below I'm not using a JSON message body.

I suspect that in general anything being inserted into an event template using `chevron` will have to be escaped.

### Steps to reproduce:

I want to generate a test SQS receive message event with the following body:

```
This message is "plain text".
It is most certainly not JSON-formatted!
```

And so I use `sam local generate-event`:

```console
$ cat body.txt
This message is "plain text".
It is most certainly not JSON-formatted!
$ sam local generate-event sqs receive-message --body "$(cat body.txt)"
{
"Records": [
{
"messageId": "19dd0b57-b21e-4ac1-bd88-01bbb068cb78",
"receiptHandle": "MessageReceiptHandle",
"body": "This message is "plain text".
It is most certainly not JSON-formatted!",
"attributes": {
"ApproximateReceiveCount": "1",
"SentTimestamp": "1523232000000",
"SenderId": "123456789012",
"ApproximateFirstReceiveTimestamp": "1523232000001"
},
"messageAttributes": {},
"md5OfBody": "786457a8a924bfb2f34d6b7fc4e2e7da",
"eventSource": "aws:sqs",
"eventSourceARN": "arn:aws:sqs:us-east-1:123456789012:MyQueue",
"awsRegion": "us-east-1"
}
]
}
```

### Observed result:

The generated JSON is invalid:

```console
$ sam local generate-event sqs receive-message --body "$(cat body.txt)" | python3 -m json.tool
Expecting ',' delimiter: line 6 column 33 (char 161)
```

To make it clear given the comments on #2848, this is nothing to do with the *body* being JSON. Irrespective of what body is used, the generated event should still be a valid JSON document.

### Expected result:

I'd expect the following JSON event:

```js
{
"Records": [
{
"messageId": "19dd0b57-b21e-4ac1-bd88-01bbb068cb78",
"receiptHandle": "MessageReceiptHandle",
"body": "This message is \"plain text\".\nIt is most certainly not JSON-formatted!",
"attributes": {
"ApproximateReceiveCount": "1",
"SentTimestamp": "1523232000000",
"SenderId": "123456789012",
"ApproximateFirstReceiveTimestamp": "1523232000001"
},
"messageAttributes": {},
"md5OfBody": "786457a8a924bfb2f34d6b7fc4e2e7da",
"eventSource": "aws:sqs",
"eventSourceARN": "arn:aws:sqs:us-east-1:123456789012:MyQueue",
"awsRegion": "us-east-1"
}
]
}
```

### Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: OS X
2. `sam --version`: SAM CLI, version 1.53.0
3. AWS region: N/A