Add support for executing groovy scripts

via the jenkins API /scriptText endpoint and return the script output if any (plain text, not json). To execute a groovy script you send a POST request to the /scriptText endpoint with Content-Type = application/x-www-form-urlencoded and a parameter named script which holds the groovy script to execute (as a string). The request returns the output of the script as a plain text. It's the same as running the script from Jenkins' Script Console