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

src/datasource/components/QueryEditor/ProblemsQueryEditor.tsx

@@ -166,6 +166,7 @@ export const ProblemsQueryEditor = ({ query, datasource, onChange }: Props) => {
             options={groupsOptions}
             isLoading={groupsLoading}
             onChange={onFilterChange('group')}
+            createCustomValue={true}
             placeholder="Group name"
           />
         </InlineField>
@@ -176,6 +177,7 @@ export const ProblemsQueryEditor = ({ query, datasource, onChange }: Props) => {
             options={hostOptions}
             isLoading={hostsLoading}
             onChange={onFilterChange('host')}
+            createCustomValue={true}
             placeholder="Host name"
           />
         </InlineField>
@@ -186,6 +188,7 @@ export const ProblemsQueryEditor = ({ query, datasource, onChange }: Props) => {
             options={proxiesOptions}
             isLoading={proxiesLoading}
             onChange={onFilterChange('proxy')}
+            createCustomValue={true}
             placeholder="Proxy name"
           />
         </InlineField>
@@ -199,6 +202,7 @@ export const ProblemsQueryEditor = ({ query, datasource, onChange }: Props) => {
               options={appOptions}
               isLoading={appsLoading}
               onChange={onFilterChange('application')}
+              createCustomValue={true}
               placeholder="Application name"
             />
           </InlineField>