Add command sets to profiler communication channel

[schmittjoseph]

Summary

This PR upgrades the profiler channel's messages to now contain a command set field to help distinguish between different receivers of the IPC message. There are currently 3 distinct command sets: ServerResponse, Profiler, and StartupHook (currently used by the HostingStartup, but this will become StartupHook-only in the future with the parameter capturing upgrades).

This change also allows multiple instances of MonitorMessageDispatcher to exist, where each is subscribed to a different command set, though this isn't done in this PR.

New IPC message format:

Field	Size (bytes)
Command set	2
Command	2
Payload size	4
Payload	%Payload size%

Other changes in this PR:

• Update Command to be unsigned.
• Update the native code to use fixed width integer types when handling the IPC messages instead of the portable types. Portable types don't guarantee the actual size of the integer, just the minimum size so it'd be technically possible for the headers buffer to be incorrectly sized on certain platforms.

Release Notes Entry