Show possible values when filtering string fields

In the old editor a user would see possible values in a dropdown when a filter expression was selected. See this video for example

[Version 9.1 query editor](https://grafana.com/docs/grafana/v9.1/datasources/postgres/#query-editor)

No repository files, tests, or entry points are identified in the issue. Start by comparing the current query editor with the linked Grafana 9.1 PostgreSQL query editor example and trace how string-field filters are rendered. Done means selecting a filter expression shows the possible values in a dropdown as in the example.

Written by the indexing model from the issue text.