feat: テーブル定義を追加

src/main/resources/db/migration/V2__Add_Block.sql

@@ -0,0 +1,8 @@
+create table if not exists blocks
+(
+    id      bigserial primary key,
+    user_id bigint not null,
+    target  bigint not null,
+    constraint fk_blocks_user_id__id foreign key (user_id) references users (id) on delete restrict on update restrict,
+    constraint fk_blocks_target_id__id foreign key (target) references users (id) on delete restrict on update restrict
+);