net: ena: Add ENA Express metrics support

ENA Express metrics, called `ena_srd` are exposed to
customers via `ethtool`.
The metrics allow customers to check the configuration
(mode), tx/rx counters as well as resource utilization.

The documentation is also updated to provide a general
explanation about ENA Express as well as links for further
information about metrics and configurations.

Signed-off-by: Igor Chauskin <igorch@amazon.com>
Signed-off-by: David Arinzon <darinzon@amazon.com>
Link: https://patch.msgid.link/20240909084704.13856-2-darinzon@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>