Add Gatling performance test suite for analytics microservice

Description

Add a Gatling performance test suite as a Maven module in the analytics microservice project. Tests cover the ingestion endpoint and analytics query endpoints under load. The suite must be runnable from CI with configurable target URL and concurrency settings.

Improvement plan: https://docs.google.com/document/d/1hVfIurPqAJJ1PN1EalmvrWXnGNt5sY21MI-409lrXpU/edit?tab=t.klqavlz4cis7#heading=h.8vek9yxw

Parent epic: #35048

Acceptance Criteria

• Gatling added as a Maven module in the analytics microservice project
• Performance scenarios cover the ingestion endpoint under load
• Performance scenarios cover analytics query endpoints under load
• Target URL and concurrency settings are configurable (env vars or Maven properties)
• Baseline results documented (throughput, p95 latency) for future comparison

Additional Context

• Source task: Task 8 (Performance tests / Gatling) from the improvement plan
• Blocked by: #35050 (ingest code refactor must be done first)
• Related branch: issue-34849-Add-dotCMS-Analytics-Proxy-Endpoint-for-/v1/analytics/-to-dot-ca-event-manager

Start by checking the analytics microservice and the prerequisite ingest refactor in #35050, then review Task 8 in the linked improvement plan. Define Gatling scenarios for the ingestion and analytics query endpoints, with configurable target URL and concurrency settings, and make them runnable in CI. Done means the Maven module runs successfully and records baseline throughput and p95 latency.