This allows us to use FIND_PACKAGE_ARGS for FetchContent with
the SYSTEM argument.

Warnings in thirdparty dependencies are not very interesting for
the development of an application. By setting the SYSTEM arg we
will include the dependency header via `-isystem` instead.

Straightforward adaption to leverage `find_package` on those systems
that have a tomlplusplus that's new enough for our purposes.

This is also relatively straight forward, except for the fact
that we don't get the glog unittest targets when the dependency
is found. Thus the changeset is slightly larger than one might
expect otherwise.

The version bump was done so that I can test it with the package
I have available on my system. Previously, the embedded msgpack
code didn't compile there. Now I simply can use the system provided
msgpack here which is much better.

This code is copied from [1] which is Apache-2.0 licensed and
owned by Meta, so I hope this is fine licensing wise.

[1]: https://github.com/facebookarchive/caffe2/blob/157ff81a12cf48eb4b0a2ad854c4ae205ad8aa07/cmake/Modules/FindRocksDB.cmake

Due to the custom build system integration for RocksDB we cannot
simply use FIND_PACKAGE_ARGS. Instead, we first do a `find_package`
and only if that fails do we fallback to the old rocksdb integration.

To better facilitate this, I took the liberty to also modernize
the CMake code a bit. Consumers of the lib just need to link
against the new `rocksdb::librocksdb` alias target to get the
required library and include directory.