feat(ci): add initial GitLab CI configuration for build and test

Add a .gitlab-ci.yml file to configure CI stages for building and testing the project. Set up caching for Maven dependencies and define build and test stages using Maven with OpenJDK 17.

.gitlab-ci.yml

+stages:
+  - build
+  - test
+
+variables:
+  MAVEN_OPTS: "-Dmaven.repo.local=$CI_PROJECT_DIR/.m2/repository"
+
+cache:
+  paths:
+    - .m2/repository
+
+before_script:
+  - export MAVEN_CLI_OPTS="--batch-mode --errors --fail-at-end --show-version"
+
+build:
+  stage: build
+  image: maven:3.8.5-openjdk-17
+  script:
+    - mvn $MAVEN_CLI_OPTS clean install
+  artifacts:
+    paths:
+      - target/*.jar
+
+test:
+  stage: test
+  image: maven:3.8.5-openjdk-17
+  script:
+    - mvn $MAVEN_CLI_OPTS test
\ No newline at end of file