ADD database integration & ADD simple dummy user entity

src/main/resources/db/schema.sql

@@ -0,0 +1,9 @@
+DROP TABLE IF EXISTS users;
+CREATE TABLE IF NOT EXISTS users (
+    id BIGINT PRIMARY KEY,
+    google_id VARCHAR(64),
+    username VARCHAR(50) NOT NULL UNIQUE,
+    email VARCHAR(100) NOT NULL UNIQUE,
+    name VARCHAR(100) NOT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);