adds new index to help support DeleteRelationships with limit

For folks running schema migrations, which typically involve
deleting relationships of one of the relations affected
in their schema, we recommend using DeleteRelationship
API with limit enabled not to overload the database in
situations where relations have millions of relationships.

DeleteRelationships with limits is fundamentally different
in that it has to do a SELECT first because Postgres does not
support DELETE ... LIMIT statements.

That particular SELECT statement was found to be unindexed
and be extremely slow in SpiceDB clusters with millions of relationships.
This commit adds a new index to support that access pattern
and make deletes+limit faster and less impactful overall.

⚠️This index uses the xid8 type and also requires Postgres 15
for the query planner to do the right thing.

internal/datastore/postgres/migrations/zz_migration.0017_add_index_tuple_alive_by_resource_rel_subject.go

@@ -0,0 +1,26 @@
+package migrations
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+)
+
+const createAliveRelByResourceRelationSubjectIndex = `CREATE INDEX CONCURRENTLY 
+	IF NOT EXISTS ix_relation_tuple_alive_by_resource_rel_subject
+	ON relation_tuple (namespace, relation, userset_namespace)
+    WHERE deleted_xid = '9223372036854775807'::xid8;`
+
+func init() {
+	if err := DatabaseMigrations.Register("add-rel-by-alive-resource-relation-subject", "add-tuned-gc-index",
+		func(ctx context.Context, conn *pgx.Conn) error {
+			if _, err := conn.Exec(ctx, createAliveRelByResourceRelationSubjectIndex); err != nil {
+				return fmt.Errorf("failed to create index for alive relationships by resource/relation/subject: %w", err)
+			}
+			return nil
+		},
+		noTxMigration); err != nil {
+		panic("failed to register migration: " + err.Error())
+	}
+}