Add support to use custom values in dropdown (#2163)

## Summary
When we switched to use `Combobox` we were no longer able to add custom
values for our dropdowns, and this prevented users from using regexp
values for `group`, `host`, `proxy` etc. This PR adds
`createCustomValue` flag so that any custom value can be entered into
the dropdowns.

## Detailed summary
- Make use of `Combobox`'s `createCustomValues` so that wherever
necessary, users are able to enter regex values.
    
<img width="720" height="221" alt="Screenshot 2026-01-05 at 7 15 23 AM"
src="https://github.com/user-attachments/assets/841716eb-cd86-49d1-b13c-a0e37e8a37b9"
/>

## Why
For users to be able to use regexp, and any custom value in our
dropdowns.

## How to test
For all query types where dropdowns are used and user configurable (i.e.
group, host, proxy, application) start typing a value that does not
already exist in the dropdown. You should see an option to `Use custom
value`

Fixes
[comment](https://github.com/grafana/grafana-zabbix/pull/2141#issuecomment-3698818476),
thanks for the testing and call out @christos-diamantis

.changeset/full-parrots-crash.md

@@ -0,0 +1,5 @@
+---
+'grafana-zabbix': minor
+---
+
+Support adding custom values in dropdowns for all query types