Csharp: `Default` value annotation

Intended UI -

```csharp
#[Table]
class Banana {
    Color color;
}

// ->
#[SpacetimeDB.Table(name = "Banana")]
class Banana {
    Color color;

    #[SpacetimeDB.Default(0)]
    uint ripeness;
}
```

here is the reference PR for rust counterpart - https://github.com/clockworklabs/SpacetimeDB/pull/3177.
And proposal - https://github.com/clockworklabs/SpacetimeDBPrivate/pull/1849/