Allow caching Jira REST API calls between rendering requests to speed up template development

See ticket: #18496

As most of the rendering time is spent with fetching data from the Jira REST API (for example, 20 sec out of 24 sec), there is no easy way to optimize the overall speed further on our side. This slows down the template development process when complex custom logic needs to be tested over and over again.

Idea: let the template developer configure rendering to keep the cache among different requests, e.g. add a "development mode -> keep cache" configuration checkbox to the UI.