Xarray-specific filter predicate pushdown. (#113)

Partition pruning: In the Xarray data model, we can always get
statistics for the dataset dims, which function like the "primary keys"
of the materialized table. These statistics lend themselves to omit
processing data that we know we don't need to. For example,

```sql
SELECT * FROM air WHERE time > '2020-02-01'
```

Will skip loading partitions whose time ranges are entirely before
2020-02-01.

Author: alxmrs

Cargo.lock

@@ -20,6 +20,7 @@ exclude = [
 [dependencies]
 arrow = { version = "57.2.0", features = ["pyarrow"] }
 async-stream = "0.3"
+async-trait = "0.1"
 datafusion = { version = "51.0.0" }
 datafusion-ffi = { version = "51.0.0" }
 futures = { version = "0.3" }