feat: permit use of system rapidyaml

[jcpunk]

This addresses the TODO to permit use of the system rapidyaml rather than the vendored files.

In theory this should make packaging a bit more consistent across various build environments.

I'd love to see this in 0.22.0

[johnbartholomew]

I think this probably won't work as-is on Debian, because the libryml-dev package doesn't contain any CMake package config (or even any pkg-config files!): https://packages.debian.org/sid/amd64/libryml-dev/filelist

Might just be a Debian packaging problem, I'm not certain.

[jcpunk]

I'll confess, I'm mostly working on Fedora/RHEL which provide:

/usr/lib64/cmake/ryml/rymlConfig.cmake
/usr/lib64/cmake/ryml/rymlConfigVersion.cmake
/usr/lib64/cmake/ryml/rymlTargets-release.cmake
/usr/lib64/cmake/ryml/rymlTargets.cmake

[johnbartholomew]

Ok. Let's assume Debian packaging will catch up eventually.