Field value suggestions should be query-aware

## What?

Field value suggestions should be "aware" of the surrounding query, such that suggested values are mostly (ideally entirely, but there are probably performance concerns) limited to valid values for the overall query.

## Why?

Valid suggestions for a given field (e.g. `filebeat_log_file_path`) may vary wildly based upon other fields in the query (e.g. `source`).

By limiting suggested values to those that are valid for the surrounding (i.e. not being modified) parts of the query, it'd be possible to get more useful suggestions. For example: only seeing the log file paths that are associated with a specific source.

## Your Environment

* Graylog Version: 4.3.0