feat: only include ppl not hosp at date of +ve test

[LindaNab]

This PR adds:

• hospital_discharge_date_before_eligible and hospital_admission_date_after_eligible to study_definition.
• Only includes people in the eligible population who haven't been hospitalised before or on elig_start or who have been hospitalised but were discharged before or onelig_start.

Notes:

• hospital_discharge_date_before_eligible and hospital_admission_date_after_eligible use covid_test_positive_date io start_date because the variable is used for excluding people from the eligible population (and not the treated but not eligible population).

[gitpod-io[bot]]

[LindaNab]

Second commit adds variable any_covid_hospital_admission_date, used to check if there was a covid hospitalisation in the 30 days prior to a +ve test (covid as a primary diagnosis or not primary). This is used alongside SGSS test data to check "As per NHSD’s test rule set logic, patients with a prior +ve test within the last 30 days were excluded".