Adding JMeter jmx file for load testing

[delatbabel]

This PR adds a JMeter file which can be used for load testing the API using Apache JMeter (https://jmeter.apache.org/)

[delatbabel]

Do you want some instructions on how to use this? It's reasonably simple but I can either add a separate instructions file or add to the existing README.md file in the api directory.

[geromegrignon]

Hello @delatbabel,

i opened a related issue to track discussions about load testing #666 (so we can focus upon technical details of the pull request here).

[Mart-Bogdan]

Why Jmeter, not Gattling?

[delatbabel]

There are hundreds of load testing tools available, I'm not going to do files for each one. JMeter is free, Gatling is not.

[Mart-Bogdan]

@delatbabel wow. That's interesting. I remember using it as CLI tool for free. As open-source tool. I don't know what Enterprise version is, I believe it allows to test from multiple servers.

My motivation was that Gattling uses textual DSL files instead of XML that could be easily source versioned.