Add support for executing additional DB migration statements

In cases where pgloader is not enough and we need to do some additional
migration work after it, we can now use
`additional_psql_statements_list` and
`additional_psql_statements_db_name`.

This is to be used when migrating `matrix-registration`'s data at the
very least.

roles/matrix-postgres/tasks/util/migrate_db_to_postgres.yml

@@ -99,6 +99,29 @@
   command:
     cmd: "mv {{ matrix_postgres_db_migration_request.src }} {{ matrix_postgres_db_migration_request.src }}.backup"
 
+- block:
+    # We can't use `{{ role_path }}` here, neither with `import_tasks`, nor with `include_tasks`,
+    # because it refers to the role that included this util, and not to the role this file belongs to.
+    - import_tasks: "roles/matrix-postgres/tasks/util/detect_existing_postgres_version.yml"
+
+    - set_fact:
+        matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_latest if matrix_postgres_detected_version_corresponding_docker_image == '' else matrix_postgres_detected_version_corresponding_docker_image }}"
+
+    - name: Execute additional Postgres SQL migration statements
+      command:
+        cmd: >-
+          {{ matrix_host_command_docker }} run
+          --rm
+          --user={{ matrix_user_uid }}:{{ matrix_user_gid }}
+          --cap-drop=ALL
+          --env-file={{ matrix_postgres_base_path }}/env-postgres-psql
+          --network={{ matrix_docker_network }}
+          {{ matrix_postgres_docker_image_to_use }}
+          psql --host=matrix-postgres --dbname={{ matrix_postgres_db_migration_request.additional_psql_statements_db_name }} --command='{{ item }}'
+      with_items: "{{ matrix_postgres_db_migration_request.additional_psql_statements_list }}"
+
+  when: "matrix_postgres_db_migration_request.additional_psql_statements_list|default([])|length > 0"
+
 - name: Inject result
   set_fact:
     matrix_playbook_runtime_results: |