feat: add an echo command to nu_plugin_example (#12754)

# Description

This PR adds a new `echo` command to the `nu_plugin_example` plugin that
simply [streams all of its input to its
output](https://github.com/nushell/nushell/pull/12754/files#diff-de9fcf086b8c373039dadcc2bcb664c6014c0b2af8568eab68c0b6666ac5ccceR47).

```
: "hi" | example echo
hi
```

The motivation for adding it is to have a convenient command to exercise
interactivity on slow pipelines.

I'll follow up on that front with [another
PR](https://github.com/cablehead/nushell/pull/1/files)

# Tests + Formatting

https://github.com/nushell/nushell/pull/12754/files#diff-de9fcf086b8c373039dadcc2bcb664c6014c0b2af8568eab68c0b6666ac5ccceR51-R55

crates/nu_plugin_example/src/commands/echo.rs

@@ -0,0 +1,55 @@
+use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
+use nu_protocol::{Category, Example, LabeledError, PipelineData, Signature, Type, Value};
+
+use crate::ExamplePlugin;
+
+/// `<list> | example echo`
+pub struct Echo;
+
+impl PluginCommand for Echo {
+    type Plugin = ExamplePlugin;
+
+    fn name(&self) -> &str {
+        "example echo"
+    }
+
+    fn usage(&self) -> &str {
+        "Example stream consumer that outputs the received input"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .input_output_types(vec![(Type::Any, Type::Any)])
+            .category(Category::Experimental)
+    }
+
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["example"]
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            example: "example seq 1 5 | example echo",
+            description: "echos the values from 1 to 5",
+            result: Some(Value::test_list(
+                (1..=5).map(Value::test_int).collect::<Vec<_>>(),
+            )),
+        }]
+    }
+
+    fn run(
+        &self,
+        _plugin: &ExamplePlugin,
+        _engine: &EngineInterface,
+        _call: &EvaluatedCall,
+        input: PipelineData,
+    ) -> Result<PipelineData, LabeledError> {
+        Ok(input)
+    }
+}
+
+#[test]
+fn test_examples() -> Result<(), nu_protocol::ShellError> {
+    use nu_plugin_test_support::PluginTest;
+    PluginTest::new("example", ExamplePlugin.into())?.test_command_examples(&Echo)
+}

crates/nu_plugin_example/src/commands/mod.rs

@@ -25,12 +25,14 @@ pub use view_span::ViewSpan;
 
 // Stream demos
 mod collect_external;
+mod echo;
 mod for_each;
 mod generate;
 mod seq;
 mod sum;
 
 pub use collect_external::CollectExternal;
+pub use echo::Echo;
 pub use for_each::ForEach;
 pub use generate::Generate;
 pub use seq::Seq;

crates/nu_plugin_example/src/lib.rs

@@ -25,6 +25,7 @@ impl Plugin for ExamplePlugin {
             Box::new(DisableGc),
             // Stream demos
             Box::new(CollectExternal),
+            Box::new(Echo),
             Box::new(ForEach),
             Box::new(Generate),
             Box::new(Seq),