Add a visual query builder for Target Filter Queries in the Management UI

[vmsh1]

Today, creating a Target Filter Query in the Management UI means typing a raw RSQL expression by hand into a plain text field, e.g.:

attribute.hardwareBoard==raspberrypi

This works, but has friction for users unfamiliar with RSQL: no autocomplete for fields, attribute keys/values, or operators, and no inline validation until save.

Idea: Add an optional guided/visual builder alongside the free-text field — build clauses via dropdowns (field → operator → value), compose them with AND/OR, and render the resulting RSQL live. Advanced users could still drop back to editing raw RSQL directly.

A few implementation notes from a quick look at the code:

• The query input is currently just a plain Vaadin TextArea/TextField (TargetFilterQueryView.java, TargetView.java) with no client-side help.
• Save calls (createFilter/updateFilter) aren't wrapped in try/catch, so invalid RSQL surfaces as a raw REST exception rather than a friendly message — worth fixing regardless of the builder.
• There's no API today to fetch distinct attribute keys/values across targets (GET /targets/{id}/attributes only returns one target's map), so a value-dropdown would need a new aggregation endpoint.

Happy to help prototype this if there's interest.