Factor out common CMake invocations into functions

[paleolimbot]

As pointed out in #719 (review) , in the changes leading up to our nice CMake build + install setup, we've accumulated quite a bit of repetition in CMakeLists.txt. If possible, we should extract out add_nanoarrow_library() and/or add_nanoarrow_test().

I seem to remember the last time I tried this that the bundling and IPC dependencies made it hard to factor out but ADBC has nice versions of both of these that seem to allow quite a bit of flexibility.

[assignUser]

I'm happy to help with this!

[paleolimbot]

That's very kind of you! I've currently hit the stage in the CMake refactoring where I never want to write CMake ever again but I'll almost certainly recover if you don't have the chance to circle back:

Some other ideas from #719 that weren't implemented there:

For packaging the shared bit could be made a component that also changes which target gets the main alias. (again something gflags does find_package(gflags COMPONENTS shared)

nit: You should be able to create an INTERFACE target for the sources and use that instead of variables: https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_SOURCES.html