rgw: AWS4 auth support for positive content-length

rgw: AWS4 auth support for positive content-length

Handle AWS4 auth on requests with positive content-length. It splits the
auth process along several steps to process the body content on the fly
instead of reading the whole body in memory. After that, it completes
the delayed AWS4 auth properly.

Requests with content-length <= 0 are validated as usual. They don't
require any kind of completion.

Requests with content-length > 0 use a streaming approach together with
a completion step.

Fixes: #10333
Signed-off-by: Javier M. Mellid <jmunhoz@igalia.com>