Hardened macOS binaries and libraries with the runtime hardened flag set on

Currently, the ruby binary coming with td-agent is not hardened
```bash
> codesign -dv /opt/td-agent/bin/ruby
Executable=/opt/td-agent/bin/ruby
Identifier=ruby-5555494463eb77d3aef0307a949c84abf11e480c
Format=Mach-O thin (x86_64)
CodeDirectory v=20400 size=614 flags=0x2(adhoc) hashes=13+2 location=system
Signature=adhoc
Info.plist=not bound
TeamIdentifier=not set
Sealed Resources=none
Internal requirements count=0 size=12
```

The runtime hardened protection on macOS ensures processes started with runtime hardened protected binaries are not inspected or altered. As an example, it is possible to start the td-agent deamon and inspect the process with `lldb`.
More on hardened runtine protection can be found [here](https://developer.apple.com/documentation/security/hardened_runtime)

It is best practice to secure binaries and all libraries used by it with the hardened runtime protection to avoid unwanted parties to inspect memory and/or alter flow of execution.
The runtime flag can be set on a binary with
```bash
codesign -f -s 'apple_developper_certificate' --option=runtime /opt/td-agent/bin/ruby
```
This will also need to be done on other libraries used by the binary (ie: `/opt/td-agent/lib/*.dylib`).

Is this something we could do as part of the build process here for td-agent on macOS?