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/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),