Add ability for a pipeline filter to completely deny a query

[converge-jrehwin]

Some filter software (such as PAIG) can either redact or deny passing a query or reply. Implementing this in a filter for a reply is simple enough, just replace the denied reply with an explanatory message. However, on a query, it would make sense to skip the rest of the processing and inform the user (ideally, also catch this during autocomplete).

I've built an implementation that does this, by having the filter return a refusal and having the endpoint handle this, along with an example PAIG filter that demonstrates it. The fix spans both pipelines and open-webui, so it would be an interesting pull request.