MINOR: proxy: Add options to drop HTTP trailers during message forwarding

MINOR: proxy: Add options to drop HTTP trailers during message forwarding

In RFC9110, it is stated that trailers could be merged with the
headers. While it should be performed with a speicial care, it may be a
problem for some applications. To avoid any trouble with such applications,
two new options were added to drop trailers during the message forwarding.

On the backend, "http-drop-request-trailers" option can be enabled to drop
trailers from the requests before sending them to the server. And on the
frontend, "http-drop-response-trailers" option can be enabled to drop
trailers from the responses before sending them to the client. The options
can be defined in defaults sections and disabled with "no" keyword.

This patch should fix the issue #2930.