HttpRequest make body only bytes

[yaythomas]

overview

Follow-up from this PR https://github.com/aws/aws-durable-execution-sdk-python-testing/pull/59/files

Refactor HttpRequest so body only is type bytes, and not dict.

acceptance criteria

1. Make HttpRequest body type bytes
2. Amend from_bytes to just update the bytes as you have it here in from_raw_bytes
3. Add a body_to_dict instance method that contains all the deserialization parsing. might need to add a _operation_name to the instance. This is what the handlers will call then, instead of "parse_json_body"