Implement amalgamation for single-file geoarrow.c/geoarrow.h distrubution

[paleolimbot]

nanoarrow goes to considerable effort to make it very, very easy to copy/paste files into another project and use them as-is. There is nothing inherent about geoarrow-c that would prevent this and I think it would be good to do this and test this early to ensure there is a clear path to "using geoarrow-c".

In this scenario it may be worth separating the kernels as optional components: not everybody needs kernels (notably: extensions that vendor geoarrow-c that implement kernels themselves).