Create handler to mutate param key names

Nobody has claimed this yet.

One common problem I've run in to working in the front-end world is that everything in javascript land is camelcased, but everything on the Crystal side is snakecased. What if we had a handler that you could enable and it would just auto-convert all param keys from `firstName` to `first_name` without you needing to change anything in either language?

```
$fetch("POST", {user: {firstName: "", lastName: ""}})

post "/users" do
params.nested(:user).get("first_name")
end
```

One major thing to consider is if your POST body is heavy (i.e. uploading a file), you don't want to parse the request body a bunch of times.

Or maybe the `get` method can just say "find a key by this or the snake cased version of this". That would probably throw things off if you wanted to pass back `first_name` and `firstName` because of some weird legacy DB thing, but then I guess you just wouldn't enable this option.

This wouldn't be something turned on by default. Just an extra handler that if you wanted, you would add to your middleware stack `Lucky::ParamTransformerHandler.new`

Start by reviewing the request middleware stack and the existing parameter parsing path; the issue proposes Lucky::ParamTransformerHandler.new but names no files or tests. Define how nested camelCase keys become snake_case, ensure request bodies are not parsed repeatedly, and verify the behavior is opt-in with coverage for the shown POST payload.

Written by the indexing model from the issue text.