Migrate configuration paths to /etc/easyhaproxy and improve health check support in E2E tests

- Refactored HAProxy configuration files, templates, and paths to use `/etc/easyhaproxy` instead of `/etc/haproxy`.
- Updated Dockerfile to generate DH params and placeholder certificates in the new configuration directory.
- Added health check support with timeout to `DockerComposeFixture` in E2E test utilities.
- Adjusted tests, templates, and plugins to use the new `Consts`-based configuration paths.
- Introduced pytest fixtures for environment isolation and temporary directory management.

build/assets/etc/easyhaproxy/haproxy/conf.d/README.md

@@ -0,0 +1,5 @@
+# Custom HAProxy Configuration
+
+Put `.cfg` files here to be included in the HAProxy configuration.
+
+These files will be available at `/etc/easyhaproxy/haproxy/conf.d/` inside the container.