Add Data Boost

[b141568]

Thank you very much for the project, this is helping a lot.

But will you consider including an option to use Data Boost?
The fact that we can run certain queries without impacting performance is amazing.

Reference:
https://cloud.google.com/spanner/docs/databoost/databoost-overview
https://cloud.google.com/spanner/docs/databoost/databoost-applications?hl=pt-br

[taka-oyama]

Yes, this is the issue for it. I'm working on it, and it should be ready sometime this month.

#113

[taka-oyama]

Closing since PR for data boost was merged #131.

Please test it out if possible and let me know what you think.

[taka-oyama]

Also, if this project is helping you, please consider giving this repo a star. Thanks.