Show tracker type in cookie tracking policy

Trackers sharing a display name can differ in type, so the generated
cookie and tracking technologies policy was ambiguous without it. Carry
the tracker type through the banner version snapshot and surface it as a
dedicated column in the policy table.

Stop the snapshot from dropping non-cookie trackers so storage, IndexedDB
and cache technologies appear in the policy and served banner config with
their real type. Duration now reflects the type when no max-age applies:
session storage clears with the tab, the remaining storage technologies
persist. Legacy snapshots predate the field and only ever held cookies,
so GetSnapshot backfills an empty type as COOKIE, keeping the non-null
GraphQL enum and policy output valid without a migration.

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

pkg/docgen/generator.go

@@ -489,6 +489,7 @@ type (
 
 	TrackerPolicyTracker struct {
 		Name     string
+		Type     string
 		Purpose  string
 		Duration string
 	}