fix: parse bytes for callback operations

[bchampp]

Issue #, if available:
#42 - pre-req for hitting it through web server.

Description of changes:
Updating the request handling for SendDurableExecutionCallbackSuccess and SendDurableExecutionCallbackFailure. Both of these requests accept their Result and Error fields as @httpPayload, meaning they get serialized to a raw bytes body instead of JSON.

I'm following the same convention as how we deserialize .from_dict() for checkpoints where we pass the execution arn from the path to .from_dict() - except using the callback id.

I tested it by using the AWS CLI and sending requests to the local server and ensuring that the requests were being deserialized properly.

Success:

aws lambda send-durable-execution-callback-success --callback-id my-callback --endpoint-url "http://localhost:5000" --result "$(echo -n 'hello world' | base64)"

...
DEBUG - Callback my-callback succeeded with result: hello world

Failure:

aws lambda send-durable-execution-callback-failure \
  --callback-id "my-callback" \       
  --error '{
    "ErrorType": "TimeoutError",
    "ErrorMessage": "Task failed due to timeout",
    "ErrorData": "Additional error context",
    "StackTrace": ["line 1", "line 2"]
  }' \
  --endpoint-url http://localhost:5000

...
DEBUG - Callback my-callback failed with error: ErrorObject(message='Task failed due to timeout', type='TimeoutError', data='Additional error context', stack_trace=['line 1', 'line 2'])

Dependencies

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