Implement CSV schema detection + fix loading from GCS emulator

[daaain]

This is a bit specific to my use case – namely loading a CSV from https://github.com/fsouza/fake-gcs-server and implementing schema detection for at least a few different basic types, including UK dates 😅

I couldn't find existing schema detection code which was a bit surprising, but maybe I just overlooked it?

But I think this could be very useful for people in a similar situation, so even if it doesn't get merged, at least people can see a solution?

Also happy to spend a bit of time on it if there are some concrete changes needed before it's mergeable! Caveat is that I'm not a Go developer, so apologies if it's not idiomatic...