Allow configuration of default max-request-body and max-response-body

[ignatevdev]

Current default values of these parameters are inappropriate for production usage, specifically max-request-body of 1G allows anyone to very easily DDOS the server kamal-proxy is running on by sending multiple concurrent requests with large payloads, even if the application only handles GET requests.

Specifying these options for each deployment works, but it would be really handy to configure those limits globally and maybe lower them by default, for example nginx client_max_body_size is only 1 MB, which is 1000 times less than the current default.

Thanks in advance.