Map Spanner DB-API onto a SQLAlchemy dialect

[tseaver]

Blocked on #3022.

Relevant SQLAlchemy docs:

• Dialect interface spec
• Existing dialects

[lukesneeringer]

In an effort to get my project management to be a bit more sane, I am going to try something else in lieu of single issues that represent large projects.

I have referenced this on a wiki page devoted to our significant feature backlog. Discussion of this particular feature is still welcome here, although I am going to close this issue until such time as this is something we take on.

[cookie223]

Hopefully comments are welcomed here. It would be great to have this and hopefully I can use Spanner as airflow backend.

[ryan-lane]

Has there been any work on this? It's a major blocker for using spanner in python.

[s5]

This is a significant blocker for me as well.

[theacodes]

@cookie223, @ryan-lane, @s5 we can't provide a full dialect until the backend service provides DML support. How useful would a read-only dialect be for your use cases?