Populate subprocessor filters from server facets

The subprocessors toolbar derived its category and region filter
options client-side from a second, unfiltered subprocessors(first: 250)
fetch, shipping up to 250 rows purely to compute two small facet sets
and silently capping the options at that limit.

Expose subprocessorCategories and subprocessorCountries on TrustCenter,
each backed by a DISTINCT query over the organization's third parties
scoped to show_on_trust_center, and read them directly in the toolbar.
The page now issues one filtered list query plus two tiny arrays, and
the options only ever include values that can actually return results.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/server/api/trust/v1/graphql/trust_center.graphql

@@ -33,6 +33,11 @@ type TrustCenter implements Node {
     filter: SubprocessorFilter
   ): SubprocessorConnection! @goField(forceResolver: true)
 
+  subprocessorCategories: [SubprocessorCategory!]!
+    @goField(forceResolver: true)
+
+  subprocessorCountries: [CountryCode!]! @goField(forceResolver: true)
+
   references(
     first: Int
     after: CursorKey