operator: Share proxy-timeout config across Envoy-related cells

`ProxyIdleTimeoutSeconds` and `ProxyStreamIdleTimeoutSeconds` are read
from the global `OperatorConfig` by three cells (ciliumenvoyconfig,
ingress, gateway-api) that all feed the same Envoy translation config.

This PR moves them into a shared `EnvoyProxyTimeouts` `cell.Config` owned
by the `ciliumenvoyconfig` cell and consumed it in the other two through
hive.

This also fixes a previous misconfiguration where those flags were not
being registered on the operator (so always defaulted to their default
value).

Signed-off-by: Hadrien Patte <hadrien.patte@datadoghq.com>

Author: HadrienPatte