Allow polars first to be used with polars group-by (#15855)

# Description
Provides functionality similar to
https://docs.pola.rs/api/python/stable/reference/dataframe/api/polars.dataframe.group_by.GroupBy.first.html
by allowing polars first to be used with a group by

```
> ❯ : [[a b c d]; [1 0.5 true Apple] [2 0.5 true Orange] [2 4 true Apple] [3 10 false Apple] [4 13 false Banana] [5 14 true Banana]] | polars into-df -s {a: u8, b: f32, c: bool, d: str} | polars group-by d | polars first | polars collect
╭───┬────────┬───┬───────┬───────╮
│ # │   d    │ a │   b   │   c   │
├───┼────────┼───┼───────┼───────┤
│ 0 │ Apple  │ 1 │  0.50 │ true  │
│ 1 │ Banana │ 4 │ 13.00 │ false │
│ 2 │ Orange │ 2 │  0.50 │ true  │
╰───┴────────┴───┴───────┴───────╯
```

Additionally, I am setting the POLARS_ALLOW_EXTENSION to true to avoid
panicking with operations using the dtype object. The conversion will
fallback to object when the type cannot be determining, so this could be
a common case.

# User-Facing Changes
- `polars first` can now be used with `polars group-by`

---------

Co-authored-by: Jack Wright <jack.wright@nike.com>

crates/nu_plugin_polars/src/dataframe/command/data/first.rs

@@ -1,6 +1,6 @@
 use crate::{
     PolarsPlugin,
-    values::{Column, CustomValueSupport, NuLazyFrame, PolarsPluginObject},
+    values::{Column, CustomValueSupport, NuLazyFrame, NuLazyGroupBy, PolarsPluginObject},
 };
 
 use crate::values::{NuDataFrame, NuExpression};
@@ -9,6 +9,7 @@ use nu_protocol::{
     Category, Example, LabeledError, PipelineData, ShellError, Signature, Span, SyntaxShape, Type,
     Value,
 };
+use polars::df;
 
 #[derive(Clone)]
 pub struct FirstDF;
@@ -87,6 +88,24 @@ impl PluginCommand for FirstDF {
                 example: "polars col a | polars first",
                 result: None,
             },
+            Example {
+                description: "Aggregate the first values in the group.",
+                example: "[[a b c d]; [1 0.5 true Apple] [2 0.5 true Orange] [2 4 true Apple] [3 10 false Apple] [4 13 false Banana] [5 14 true Banana]] | polars into-df -s {a: u8, b: f32, c: bool, d: str} | polars group-by d | polars first | polars sort-by [a] | polars collect",
+                result: Some(
+                    NuDataFrame::new(
+                        false,
+                        df!(
+                            "d" => &["Apple", "Orange", "Banana"],
+                            "a" => &[1, 2, 4],
+                            "b" => &[0.5, 0.5, 13.0],
+                            "c" => &[true, true, false],
+
+                        )
+                        .expect("dataframe creation should succeed"),
+                    )
+                    .into_value(Span::test_data()),
+                ),
+            },
         ]
     }
 
@@ -106,6 +125,9 @@ impl PluginCommand for FirstDF {
             PolarsPluginObject::NuLazyFrame(lazy) => {
                 command_lazy(plugin, engine, call, lazy).map_err(|e| e.into())
             }
+            PolarsPluginObject::NuLazyGroupBy(groupby) => {
+                command_groupby(plugin, engine, call, groupby).map_err(|e| e.into())
+            }
             _ => {
                 let expr = NuExpression::try_from_value(plugin, &value)?;
                 let expr: NuExpression = expr.into_polars().first().into();
@@ -146,6 +168,20 @@ fn command_lazy(
     res.to_pipeline_data(plugin, engine, call.head)
 }
 
+fn command_groupby(
+    plugin: &PolarsPlugin,
+    engine: &EngineInterface,
+    call: &EvaluatedCall,
+    groupby: NuLazyGroupBy,
+) -> Result<PipelineData, ShellError> {
+    let rows: Option<usize> = call.opt(0)?;
+    let rows = rows.unwrap_or(1);
+    let res = groupby.to_polars().head(Some(rows));
+    let res: NuLazyFrame = res.into();
+
+    res.to_pipeline_data(plugin, engine, call.head)
+}
+
 #[cfg(test)]
 mod test {
     use super::*;
@@ -153,6 +189,10 @@ mod test {
 
     #[test]
     fn test_examples() -> Result<(), ShellError> {
+        // // Extensions are required for the group-by functionality to work
+        // unsafe {
+        //     std::env::set_var("POLARS_ALLOW_EXTENSION", "true");
+        // }
         test_polars_plugin_command(&FirstDF)
     }
 }

crates/nu_plugin_polars/src/main.rs

@@ -4,6 +4,13 @@ use nu_plugin_polars::PolarsPlugin;
 fn main() {
     env_logger::init();
 
+    // Set config options via environment variable
+    unsafe {
+        // Extensions are required for certain things like aggregates with object dtypes to work
+        // correctly. It is disabled by default because of unsafe code.
+        // See https://docs.rs/polars/latest/polars/#user-guide for details
+        std::env::set_var("POLARS_ALLOW_EXTENSION", "true");
+    }
     match PolarsPlugin::new() {
         Ok(ref plugin) => serve_plugin(plugin, MsgPackSerializer {}),
         Err(e) => {