Almost standard now-a-days... (and You get it automatically in cloud)
Some Apps, like JSU App and Script Runner App offer parts of automation
Can be "integrated via shell calls from script, powershell and other programming languages.
As it is java based, it is virtually platform independant, the main downside is that is is slow.
See Atlassian (old) samples at: https://confluence.atlassian.com/display/JIRA/Jelly+Escalation
|Groovy||Supported by the Script Runner App and some other Apps.|
See some samples at library.adaptavist.com
|REST API||Interfacing with JIRA through webservices - REST, The SOAP and XML-RCP is deprecated|