:art: add dbmate demo

rpc/tulip/tulip/db/.env.orig

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+DATABASE_URL="postgres://www:change-me@127.0.0.1:5432/tulip?sslmode=disable"

rpc/tulip/tulip/db/.gitignore

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+/.env
+/db/schema.sql

rpc/tulip/tulip/db/README.md

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+# Usage
+
+```bash
+# setup your database profile
+cp .env.orig .env
+
+# creating a migration
+dbmate new XXX
+# run all migrations
+dbmate up
+# rollback a migration
+dbmate rollback
+# show current status
+dbmate status
+```

rpc/tulip/tulip/db/db/migrations/20231109065959_create_t1.sql

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+-- migrate:up
+create table t1(id integer);
+
+-- migrate:down
+drop table t1;
+

rpc/tulip/tulip/db/db/migrations/20231109070013_create_t2.sql

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+-- migrate:up
+create table t2(id integer);
+
+-- migrate:down
+drop table t2;
+