Add helper class to generate schemas directly

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Description

Schemas currently have to be written manually by user, [example](https://github.com/ververica/ververica-fluss-examples/blob/main/fluss-food-delivery/src/main/java/com/ververica/models/FoodDeliveryDomain.java#L50):

```
return Schema.newBuilder()
.column("orderId", DataTypes.STRING())
.column("courierId", DataTypes.STRING())
.column("restaurantId", DataTypes.STRING())
.column("destinationLatitude", DataTypes.DOUBLE())
.column("destinationLongitude", DataTypes.DOUBLE())
.column("estimatedPrepTimeSeconds", DataTypes.BIGINT())
.column("estimatedTravelTimeSeconds", DataTypes.BIGINT())
.column("eventTimestamp", DataTypes.TIMESTAMP())
.build();
```

We can improve user experience by introducing helper method for this. For example, a method can be added under `Schema`

```
Schema.fromClass(Class class)
```

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start with the Schema API and the example class FoodDeliveryDomain.java linked in the issue. Determine the intended behavior and supported class-to-schema mapping for Schema.fromClass; done means a helper can generate the equivalent schema without manually declaring each column.

Written by the indexing model from the issue text.