Prototoype for Datasette on PostgreSQL

I thought this would never happen, but now that I'm deep in the weeds of running SQLite in production for Datasette Cloud I'm starting to reconsider my policy of only supporting SQLite.

Some of the factors making me think PostgreSQL support could be worth the effort:

• Serverless. I'm getting increasingly excited about writable-database use-cases for Datasette. If it could talk to PostgreSQL then users could easily deploy it on Heroku or other serverless providers that can talk to a managed RDS-style PostgreSQL.
• Existing databases. Plenty of organizations have PostgreSQL databases. They can export to SQLite using db-to-sqlite but that's a pretty big barrier to getting started - being able to run datasette postgresql://connection-string and start trying it out would be a massively better experience.
• Data size. I keep running into use-cases where I want to run Datasette against many GBs of data. SQLite can do this but PostgreSQL is much more optimized for large data, especially given the existence of tools like Citus.
• Marketing. Convincing people to trust their data to SQLite is potentially a big barrier to adoption. Even if I've convinced myself it's trustworthy I still have to convince everyone else.
• It might not be that hard? If this required a ground-up rewrite it wouldn't be worth the effort, but I have a hunch that it may not be too hard - most of the SQL in Datasette should work on both databases since it's almost all portable SELECT statements. If Datasette did DML this would be a lot harder, but it doesn't.
• Plugins! This feels like a natural surface for a plugin - at which point people could add MySQL support and suchlike in the future.

The above reasons feel strong enough to justify a prototype.

No files, tests, or entry points are named. Start by reviewing Datasette's existing SQLite support and the plugin architecture, then define the smallest PostgreSQL prototype that can accept a PostgreSQL connection string; done should mean the prototype works against a PostgreSQL database and its scope is documented.