Add post request call for the opbeans-ruby project

The opbeans-ruby app has a post endpoint `api/orders#create`

Currently, there is a secario for sending a request to a post endpoint for python and go, but not for Ruby. ([ref here](https://github.com/elastic/opbeans-loadgen/blob/8435d472ac515fa13505fd117397a16fa1436027/molotov_scenarios.py#L113-L136))

I'd like to suggest that the Ruby post endpoint be requested as well. I was interested in this when testing the `capture_body` config option with `apm-integration-testing`.

Start in molotov_scenarios.py, particularly the referenced Python and Go POST-request scenarios. Compare those scenarios with the opbeans-ruby endpoint api/orders#create and add the corresponding Ruby request scenario. Done means the load generator requests that Ruby POST endpoint alongside the existing language-specific scenarios.

Written by the indexing model from the issue text.