Add a diesel and diesel-async example (#1853)

Co-authored-by: David Pedersen <[email protected]>

examples/diesel-async-postgres/migrations/2023-03-14-180127_add_users/down.sql

@@ -0,0 +1,2 @@
+-- This file should undo anything in "up.sql"
+DROP TABLE "users";

examples/diesel-async-postgres/migrations/2023-03-14-180127_add_users/up.sql

@@ -0,0 +1,6 @@
+-- Your SQL goes here
+CREATE TABLE "users"(
+    "id" SERIAL PRIMARY KEY,
+    "name" TEXT NOT NULL,
+    "hair_color" TEXT
+);