flb_utils: add code for getting the machineID on macOS.

[pwhelan]

Summary

Add code to get the machineID (serial number) on macOS machines.

This value is used after hashing it to identify agents in a fleet that does not have a machineID already assigned manually with the MachineID configuration property.

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[edsiper]

@pwhelan merged, thanks. Please note that prefix must not have the flb_ prefix.