Adhoc filters: Fix regression with displaying groups

fixes the display of groups in adhoc filters
we initially regressed this with https://github.com/grafana/scenes/pull/766
this PR does a couple of things:
- creates a separate getAdhocOptionSearcher instead of trying to reuse the generic getOptionSearcher which is typed for VariableOptions
- ~will add some tests for this as well~ ✅
- moves the handleOptionGroups logic to AdHocFilterRenderer
- so the flow is something like:
- opening the option menu calls model._getValuesFor
- this in turn calls ds.getTagValues, which returns a MetricFindValue[] array containing an optional group parameter
- _getValuesFor then maps this to SelectableValue[] but keeps the group parameter
- this is set in state as values in AdHocFilterRenderer
- AdHocFilterRenderer then uses the new option searcher to filter these options down, and calls handleOptionGroups to group them all together as before

you can test this locally before/after by adjusting the adhoc filters demo and providing a custom getTagValuesProvider:

              getTagValuesProvider: async () => ({
                replace: true,
                values: [{
                  text: 'Alice',
                  value: 'alice',
                  group: 'People',
                }, {
                  text: 'Bar',
                  value: 'bar'
                }, {
                  text: 'Cat',
                  value: 'cat',
                  group: 'Animals',
                }, {
                  text: 'Bob',
                  value: 'bob',
                  group: 'People',
                }, {
                  text: 'Dog',
                  value: 'dog',
                  group: 'Animals',
                }, {
                  text: 'Foo',
                  value: 'foo',
                }]
              })

note: there is a minor styling issue where we're not showing boundaries between groups for virtualized Selects. this needs fixing separately in core.
- you can see it in the canary storybook as well
note 2: ~working on a test to prevent future regressions now 🤓~ there is already a test for this in AdhocFiltersVariable.test.ts, but it's currently skipped and can't be enabled until scenes moves to @grafana/ui@11.1.0

📦 Published PR as canary version: 5.3.6--canary.815.9779024969.0

:sparkles: Test out this PR locally via: ```bash npm install @grafana/scenes-react@5.3.6--canary.815.9779024969.0 npm install @grafana/scenes@5.3.6--canary.815.9779024969.0 # or yarn add @grafana/scenes-react@5.3.6--canary.815.9779024969.0 yarn add @grafana/scenes@5.3.6--canary.815.9779024969.0 ```

grafana / scenes

Adhoc filters: Fix regression with displaying groups #815