[ci] Commit PR based Java unit tests for Buildkite (#15437) (#15441)

This commit adds Java unit tests (including sonar scans) to the
Buildkite pull request pipeline.

Relates: https://github.com/elastic/ingest-dev/issues/1721
(cherry picked from commit e285425d54)

Co-authored-by: Dimitrios Liappis <dimitrios.liappis@gmail.com>

.buildkite/pull_request_pipeline.yml

@@ -28,3 +28,17 @@ steps:
 
       source .buildkite/scripts/common/container-agent.sh
       ci/unit_tests.sh ruby
+
+  - label: ":java: Java unit tests"
+    key: "java-unit-tests"
+    agents:
+      image: "docker.elastic.co/ci-agent-images/platform-ingest/buildkite-agent-logstash-ci"
+      cpu: "8"
+      memory: "16Gi"
+      ephemeralStorage: "100Gi"
+    command: |
+      set -euo pipefail
+
+      source .buildkite/scripts/common/container-agent.sh
+      source .buildkite/scripts/pull-requests/sonar-env.sh
+      ci/unit_tests.sh java

.buildkite/scripts/pull-requests/sonar-env.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+SONAR_TOKEN_PATH="kv/ci-shared/platform-ingest/elastic/logstash/sonar-creds"
+export SONAR_TOKEN=$(retry -t 5 -- vault kv get -field=token ${SONAR_TOKEN_PATH})
+
+export SOURCE_BRANCH=$GITHUB_PR_BRANCH
+export TARGET_BRANCH=$GITHUB_PR_TARGET_BRANCH
+export PULL_ID=$GITHUB_PR_NUMBER
+export COMMIT_SHA=$BUILDKITE_COMMIT