Make all parameters snake_case regardless of what the OpenAPI schema says

[phalt]

For headers, we reformat the keys from hello_header to hello-header. However, all other parameters treat the open api schema as the source of truth, which means you can get all sorts of whacky formatting on them.

If we want to keep them pythonic, we could underscore case everything and just handle formatting in the schema itself.

If this gets enough 👍 I can make that change.