Change PluginCommand API to be more like Command (#12279)

# Description

This is something that was discussed in the core team meeting last
Wednesday. @ayax79 is building `nu-plugin-polars` with all of the
dataframe commands into a plugin, and there are a lot of them, so it
would help to make the API more similar. At the same time, I think the
`Command` API is just better anyway. I don't think the difference is
justified, and the types for core commands have the benefit of requiring
less `.into()` because they often don't own their data

- Broke `signature()` up into `name()`, `usage()`, `extra_usage()`,
`search_terms()`, `examples()`
- `signature()` returns `nu_protocol::Signature`
- `examples()` returns `Vec<nu_protocol::Example>`
- `PluginSignature` and `PluginExample` no longer need to be used by
plugin developers

# User-Facing Changes
Breaking API for plugins yet again 😄

crates/nu_plugin_custom_values/src/update_arg.rs

@@ -1,15 +1,22 @@
 use crate::{update::Update, CustomValuePlugin};
 use nu_plugin::{EngineInterface, EvaluatedCall, SimplePluginCommand};
-use nu_protocol::{Category, LabeledError, PluginSignature, SyntaxShape, Value};
+use nu_protocol::{Category, LabeledError, Signature, SyntaxShape, Value};
 
 pub struct UpdateArg;
 
 impl SimplePluginCommand for UpdateArg {
     type Plugin = CustomValuePlugin;
 
-    fn signature(&self) -> PluginSignature {
-        PluginSignature::build("custom-value update-arg")
-            .usage("PluginSignature for a plugin that updates a custom value as an argument")
+    fn name(&self) -> &str {
+        "custom-value update-arg"
+    }
+
+    fn usage(&self) -> &str {
+        "Updates a custom value as an argument"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
             .required(
                 "custom_value",
                 SyntaxShape::Any,