Minimal emisv2 backend

[rebkwok]

The absolute minimum needed to create an EMIS V2 backend that can be used to run the smoketest dataset definition against the EMIS test database.

To test the connection:

ehrql test-connection --backend emisv2 --url trino://<username>:<token>@explorerplus.stagingemisinsights.co.uk:443/hive/explorer_open_safely

To run the smoketest dataset definition:

ehrql generate-dataset tests/acceptance/external_studies/test-age-distribution/analysis/dataset_definition.py --backend emisv2 --query-engine trino --dsn trino://<username>:<token>@explorerplus.stagingemisinsights.co.uk:443/hive/explorer_open_safely

We don't have permission to create tables in the test db, hence the change to the Trino query engine; we should assume we will have that permission eventually, so that's just a workaround to get it working.

The patient table in the test db also has no date_of_birth values (it's null for every patient), so if you want to see it return anything from the smoketest dataset definition, you'll need to modify the population definition there.

[rebkwok]

Note: you need this (atm) in order to try running ehrql against the staging db locally. We don't want to put it into the actual code because we expect to to be able to create tables, so we want to run tests using the create-tables code. (We might need some modification in order to create them in another schema.)

[alarthast]

Moved the codebase changes to #2746; I did not repeat the instructions for local connection there (but I verified that they work for me too).