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ARROW-9836: [Rust][DataFusion] Improve API for usage of UDFs

See associated issue and document for details.

The gist is that currently, users call UDFs through

```
df.select(scalar_functions(“my_sqrt”, vec![col(“a”)], DataType::Float64))
```

and this PR proposes a change to

```
let functions = df.registry()?;

df.select(functions.udf(“my_sqrt”, vec![col(“a”)])?)
```

so that they do not have to remember the UDFs return type when using it (and a whole lot other things for us internally).

Closes #8032 from jorgecarleitao/registry

Authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>