Feature: Add build options, allow linking to system GDAL

Now that the build setup doesn't suck again (at least so far so good) we should allow people building from source to choose whether or not they want to statically link with the dependencies vendored by the extension or instead link with their system libs. This is particularly useful in the case of GDAL which supports a lot more drivers than I expect we will ever bundle completely.

Also:
- We should allow people to opt out from gdal completely. We either add some sort of `#ifdef` or invert the control flow for registering modules so that we can leave out the gdal code from the compilation smoothly by just not including the directory in cmake. This would unblock #17.
- Maybe we shouldn't vendor the dependencies at all but instead download them from github on-demand directly using ExternalProjectAdd. I think everything we use is on github already so this should be fine?