Extend issue view and search request views to be able to produce binary content

At the moment the Issue Views and hence Search Request Views can only produce String content.

This protects them against making mistakes with encoding, but does not allow to produce binary file formats, such as PDF.

We need to extend this plugin point to optionally be able to produce binary output. I think this needs to be optional, so if the plugin only needs to produce text, the plugin writer should not have to worry about encoding.