Add attribute to specify an issue template while creating an issue via API

Problem to solve

Specify an issue template to use while creating an issue via API.

Further details

Currently, we let users specify an issue description while creating an issue via API. However, in the UI one also gets the option to use a template instead. Can we expose this option in the API?

Proposal

Let users specify a template file name as an attribute to the new issue API endpoint. Fill in content of that file as issue description if the file is found. If the file is not found, create an issue with empty description.