When recieving large payload objects, the server can often slowdown
or get fully exhausted if bunch of requests with large payload body size
come in. When request with large payload come, lot of the time is spent
reading it into then, writing it to the IO for rack, before the request
is passed to the rails app for further processing.

While there are some workarounds around limiting large request sizes, like at nginx layer
by setting `client_max_body_size`, which would return a `413` back to the client,
today that is not possible with puma.

This would be a very nice feature to have, especially when there is no reverse
proxy in between client and server.

This approach - allows a user to set `http_content_length_limit_exceeded`
via  a config variable (defaults to `nil`). This value is then compared
against `Content-Length` http header before reading the body into buffer.
If the user value is higher than the header value, the request body is
not loaded and an immediate `413` (`Payload too large`) http response is returned,
from `Puma::Request.handle_request`.

Without having to buffer in the huge request and return the `413`
immediately to the clients that send a `Content-Length` - is a nice feature
and helpful protection to have.

http header is present.

Co-authored-by: Nate Berkopec <nate.berkopec@gmail.com>