Add GitLab CI using the Drupal Association templates, with PostGIS support for tests

Problem/Motivation

The module has no .gitlab-ci.yml, so no pipeline runs on commits or merge requests. Coding standards, static analysis and the kernel test suite can currently only be run locally, which means every contribution has to be verified by hand before it is merged. For example, the tests added in #3625529 were only run in a local environment.

The module requires PostgreSQL with the PostGIS extension, and all kernel tests depend on it. A default CI setup running on MySQL/MariaDB or plain PostgreSQL would therefore not be able to run the test suite.

Steps to reproduce

1. Open a merge request against 1.0.x.
2. No pipeline is triggered; no phpcs, phpstan or PHPUnit results are reported on the MR.

Proposed resolution

Add a .gitlab-ci.yml based on the Drupal Association's GitLab CI templates (include.drupalci.main.yml, include.drupalci.variables.yml, include.drupalci.workflows.yml), so that pipelines run on pushes and merge requests.

• Set _TARGET_DB_TYPE to pgsql so the tests run against PostgreSQL.
• Make sure the database service used by the PHPUnit job has PostGIS available and the extension enabled. If the template's PostgreSQL image does not ship PostGIS, override the database service with a PostGIS image (for example postgis/postgis) of a matching PostgreSQL version.
• Keep the template's default jobs: composer, phpcs, phpstan, cspell, eslint/stylelint (where applicable) and PHPUnit.
• Fix any phpcs, phpstan or cspell findings on existing code that would make the pipeline fail, or document them in the CI configuration where fixing them is out of scope.

Remaining tasks

• Verify whether the template's PostgreSQL image includes PostGIS.
• Add .gitlab-ci.yml with the PostGIS-enabled database service.
• Get a green pipeline on 1.0.x.
• Review.

User interface changes

None.

API changes

None.

Data model changes

None.