Introduce a command line arg to delay agent exit

Summary:
Certains tests, e.g. `forceIsolateDuringWarmBoot()` require that an operation (firmware isolate) happen while the cold boot binaries have unregistered SDK callbacks but warmboot binaries have not started.

In mono, this is simple, as the test can add additional delay after sdk callbacks for mono binary are unregistered

https://www.internalfb.com/code/fbsource/[2f9795f95bf2217920103973faf3977373ccbb12]/fbcode/fboss/agent/test/agent_hw_tests/AgentVoqSwitchFirmwareTests.cpp?lines=255-258

This approach doesn't work for multi-switch mode, as the test binary where the `std::atexit()` runs doesn't control the h/w agent. The h/w agent is stopped afterwords and it's callbacks are not unregistered by the the time fw isolation happens.

Solution
=====
Introduce a command line arg, that will be set by the test-fixture to delay h/w agent exiting in multi-switch mode or monolith-agent in mono mode

NOTE: I had implemented (diff stack D74945168) a netcastle based delay between s/w and h/w agents for such tests. However, shri-khare and jasmeetbagga mentioned that it won't work for OSS. This new logic is jasmeetbagga's suggestion.

Differential Revision: D75363390

fbshipit-source-id: 0835e6a37a3b7886233539db798aae37491040ab

Author: Ashutosh Grewal

fboss/agent/AgentFeatures.cpp

@@ -306,3 +306,5 @@ DEFINE_bool(
     update_route_with_dlb_type,
     false,
     "Flag to perform a DLB type update in FIB state");
+
+DEFINE_int32(agent_exit_delay_s, 0, "Delay in seconds before the agent exits");

fboss/agent/AgentFeatures.h

@@ -104,3 +104,4 @@ DECLARE_int32(ecmp_resource_manager_make_before_break_buffer);
 
 DECLARE_int32(update_stats_interval_s);
 DECLARE_bool(update_route_with_dlb_type);
+DECLARE_int32(agent_exit_delay_s);