[Nexthop] Implement distributed command registration system

[manoharan-nexthop]

Summary

Currently the CommandTree is statically built with all the commands and their actions defined in a single file CmdListConfig.cpp / CmdList.cpp. This makes it difficult to maintain over time and with all the commands included in a single file, makes the compilation slow.

Proposed mechanism:
Each CLI will do the registration of the CLI statically from within its definition file. As an Example,

   namespace {
   facebook::fboss::CommandDef<facebook::fboss::CmdConfigVlanStaticMac>
       registerConfigVlanStaticMac(
           "config vlan static-mac",
           "Configure VLAN static MAC");
   } // namespace

When the above is done, the CommandDef class invokes CommandRegistry::registerCommand() to add the command to a vector at the start of the CLI process.

And when the CLI initialization is done invoking kCommandTree(), it invokes CommandRegistry::buildTree() to create the sorted tree by splitting the command string and creating nodes for them.

Since the commands are registered (added to vector at random order), config vlan static-mac could come before config vlan, the buildTree takes care of handling them gracefully based on the handler registration information for each CLI.

There are 3 registration types that are introduced:

Type	Use Case	Handlers
CommandDef	Commands with handlers (most commands)	Auto-inferred from CmdType
CommandDefNoHandler	Leaf commands with no children and no handler	None
AttributeDef	Simple attributes handled by parent	Only argTypeHandler

NOTE: Parent commands (commands with children) must use CommandDef (as it is required to handle the depth counter in argument parsing).

Test Plan

1. All the Unit tests passes
2. Ran the CLI end to end tests and they pass.