BUG/MEDIUM: h3: trim whitespaces in header value prior to QPACK encoding

BUG/MEDIUM: h3: trim whitespaces in header value prior to QPACK encoding

This commit does a similar job than the previous one, but it acts now on
the response path. Any leading or trailing whitespaces characters from a
HTX block header value are removed, prior to the header encoding via
QPACK.

This must be backported up to 2.6.

(cherry picked from commit bd3587574dae43cc019d8e9998e0996c75550de4)
Signed-off-by: Aurelien DARRAGON <adarragon@haproxy.com>
(cherry picked from commit a4f7bfa029ada954c8fae75c1426555a3a4012d4)
[ada: ctx adjt]
Signed-off-by: Aurelien DARRAGON <adarragon@haproxy.com>