fboss2 cli: show environment sensor: Refactor into .h and .cpp

Summary:
Previously, any change in the .h file caused a compilation step taking
2-3 mins. Now, every change in the .h file still takes the same 2-3 mins, but
these changes are rare. Changes to the .cpp file (much more frequent) take
30-40 sec.

Reviewed By: Scott8440

Differential Revision: D62660328

fbshipit-source-id: 61827dda83344269fdced710c28cf86a2f14b238

fboss/cli/fboss2/BUCK

@@ -271,6 +271,7 @@ cpp_library(
         "commands/show/facebook/bgp/ribpolicy/CmdShowBgpRibPolicyUtils.cpp",
         "commands/show/facebook/config/CmdShowConfigUtils.cpp",
         "commands/show/facebook/environment/env_keys.cpp",
+        "commands/show/facebook/environment/sensor/CmdShowEnvironmentSensor.cpp",
         "commands/show/facebook/fsdb/CmdShowFsdbUtils.cpp",
         "commands/show/route/utils.cpp",
         "facebook/CmdHandlerImpl.cpp",