Add JMeter Performance Test for Spring PetClinic REST API

[mo-beck]

This PR introduces a JMeter performance test for benchmarking the scalability of the Spring PetClinic REST API.

• Implements CRUD performance tests for owners, pets, and visits.
• Works with the default database configuration, ensuring consistency in local tests.
• Adds a README with clear instructions for running tests and analyzing results.

Notes:
The test supports configurable parameters (threads, duration, ramp-up time, operations) for flexibility across different environments.

[mo-beck]

Thanks. I will send another PR with the recommendations.

[arey]

Thank you Monica for doing such a great job.