add cmake options to force vt to use external libraries

[nmm0]

What Needs to be Done?

Package managers typically do not work well with bundled libraries, potentially because of version conflicts, build efficiency, and so on. We should provide a set of flags to force the use of find_package for some external libraries. These should be off by default, but potentially enabled in the spack build.

It's worth noting we may have to figure out our namespacing issue since we currently bundle libs ilke fmt under fmt::vt

[cz4rs]

As discussed today, this would probably mean reverting all the renaming fixes and having "vanilla" fmt copy again (which I think is nice).
In theory we could even use find_package by default and resort to CMAKE_DISABLE_FIND_PACKAGE_... for forcing the embedded copy.

Anyways, since this is a low priority for now, we can also wait for C++20 and std::format adoption 😉