Link for Later Service as a Standalone server using axum

Development

For local development, a standalone server using axum can be used with either an In-memory database or a MongoDb repository.

To use In-memory database, set INMEMORY_DB before running the server
```
INMEMORY_DB=true cargo run --bin link-for-later
```

To use MongoDb, set the MongoDB server and database name before running the server

MONGODB_URI="mongodb://localhost:27017" MONGODB_DATABASE_NAME="test" cargo run --bin link-for-later

You will be able to send requests to the server using port 8080.