Initial version of bigquery backend.

First version of bigquery backend.

Lot of TODOs. But it should be good enough for anyone who wants to try
it out. Marked experimental.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
https://github.com/SeanDolphin/bqschema/blob/master/toStructs.go has an example.
The downside is that we need to keep the structure relatively flat and simple.