meta-gbmc-staging: Make nvmesensor metric store configurable

Introduce a systemd drop-in override for xyz.openbmc_project.nvmesensor
to dynamically control the metric store functionality at runtime.
This commit updates the nvmed.bb recipe to deploy an override.conf
that evaluates the presence of enable.flag or disable.flag during
ExecStart. Also, SRCREV bump to ca96e2fc78

platform15: http://fusion2/0d0c7dff-16d2-3caa-86e3-55bb5c6fcac0, Result: pass
platform11: http://fusion2/1970c041-f822-38a1-a200-0cf9e07a64c5, Result: pass

Platforms-Affected: All w/ NVMe
Tested: N/A
Google-Bug-Id: 514372983
Change-Id: I9d45bc94352c5969542e81c5dad93b1556d786cf
Signed-off-by: Muhammad Usama <muhammadusama@google.com>
(cherry picked from commit 3838eec9b67afdded28836f2a451577fc2f6d17e)