quadlet: add HttpProxy option for Container sections

Add support for HttpProxy key in quadlet Container sections to control
proxy environment variable inheritance during image pulls and builds.

- HttpProxy=true enables proxy inheritance (default podman behavior)
- HttpProxy=false disables proxy inheritance
- When omitted, uses podman's default behavior

This addresses the need for declarative proxy configuration in IPv6-only
networks and other scenarios where proxy settings need to be controlled
at the container level without manual workarounds.

Fixes #26925

Signed-off-by: Ondřej Gajdušek <ogajduse@redhat.com>

test/e2e/quadlet_test.go

@@ -901,6 +901,8 @@ BOGUS=foo
 		Entry("group-add.container", "group-add.container"),
 		Entry("health.container", "health.container"),
 		Entry("host.container", "host.container"),
+		Entry("httpproxy-false.container", "httpproxy-false.container"),
+		Entry("httpproxy-true.container", "httpproxy-true.container"),
 		Entry("hostname.container", "hostname.container"),
 		Entry("idmapping.container", "idmapping.container"),
 		Entry("image.container", "image.container"),