feat: implement User and UserCredential models with repository and service layers

src/main/resources/db/migration/V4__user.sql

@@ -0,0 +1,16 @@
+DROP TABLE IF EXISTS app_user;
+CREATE TABLE app_user
+(
+    id       BIGSERIAL    NOT NULL PRIMARY KEY,
+    username VARCHAR(255) NOT NULL,
+    email    VARCHAR(255) NOT NULL
+);
+
+DROP TABLE IF EXISTS app_user_credential;
+CREATE TABLE app_user_credential
+(
+    user_id    BIGINT       NOT NULL REFERENCES app_user (id),
+    provider   VARCHAR(255) NOT NULL,
+    credential VARCHAR(255) NOT NULL,
+    CONSTRAINT app_user_credential_pkey PRIMARY KEY (user_id, provider)
+);