Fully support EMIS backend

[rebkwok]

#797 added the main tables to support EMIS as a backend. There is still some work to do before studies can be fully runnable in EMIS:

• authentication
  • EMIS uses TLS certs (cohort-extractor implementation)
  • https://trino.io/docs/current/security/tls.html
• trino version
  • we're using a different trino base docker image in ehrQL tests to the one used in cohort-extractor. This is so we can use the memory connector in tests, but we should verify that core ehrQL functionality is consistent with the version of trino that EMIS uses (cohort-extractor is still using a presto db URL and the presto-python-client rather than trino
• Data checks: some of the data in EMIS looks similar but different to TPP. In cohort-extractor, there appear to be some assumptions that it is the same (e.g. returning values are specified using the same names, such as nuts1_region_name, but actually contain different value ranges). Ones I noticed while working on Add an EMIS schema and backend #797 are:
  • patient_all_orgs_v2.english_region_name is used to return nuts1_region_name in cohort-extractor, but values do not match names of NUTS1 regions
  • MSOA in EMIS contains Scottish and Welsh MSOAs (although registered practices for these patients is in England)
• Test that a minimal study can run in EMIS (this is a carry-over from Add an EMIS schema and backend #797)