Add an expand option to return plain text in the REST API for wiki fields

Issue Summary

We would like to be able to get the plain text of a wiki field.
Using the /rest/api/2/issue/<issueId>?fields=<field>&expand=renderedFields, we can get an HTML result from the wiki field content, however, we would need to work this result using a parser like html2text, for instance, to get the plain text.

We need another expand option that could return the plain text of a wiki field.

Steps to Reproduce

• Create a new Issue
• At the description field, add some test using markup options like bold, italic, bullets, etc
• Call the REST API to get the content of this field
  http://l<jira base url>/rest/api/2/issue/PS74995-3?fields=description&expand=renderedFields

Expected Results

• Get the plain text of the description to be used somewhere else

Actual Results

• It is not possible to get the plain text, only the markup content or a rendered result in HTML

Workaround

• Use a tool like html2text to parse the results