Show plugin extra usage and search terms (#10952)

# Description

The `PluginSignature` type supports extra usage but this was not
available in `plugin_name --help`. It also supports search terms but
these did not appear in `help commands`

New behavior show below is the "Extra usage for nu-example-1" line and
the "Search terms:" line

```
❯ nu-example-1 --help
PluginSignature test 1 for plugin. Returns Value::Nothing

Extra usage for nu-example-1

Search terms: example

Usage:
  > nu-example-1 {flags} <a> <b> (opt) ...(rest)

Flags:
  -h, --help - Display the help message for this command
  -f, --flag - a flag for the signature
  -n, --named <String> - named string

Parameters:
  a <int>: required integer value
  b <string>: required string value
  opt <int>: Optional number (optional)
  ...rest <string>: rest value string

Examples:
  running example with an int value and string value
  > nu-example-1 3 bb
```

Search terms are also available in `help commands`:

```
❯ help commands | where name == "nu-example-1" | select name search_terms
╭──────────────┬──────────────╮
│     name     │ search_terms │
├──────────────┼──────────────┤
│ nu-example-1 │ example      │
╰──────────────┴──────────────╯
```

# User-Facing Changes

Users can now see plugin extra usage and search terms 

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu_plugin_example/src/nu/mod.rs

@@ -10,6 +10,8 @@ impl Plugin for Example {
         vec![
             PluginSignature::build("nu-example-1")
                 .usage("PluginSignature test 1 for plugin. Returns Value::Nothing")
+                .extra_usage("Extra usage for nu-example-1")
+                .search_terms(vec!["example".into()])
                 .required("a", SyntaxShape::Int, "required integer value")
                 .required("b", SyntaxShape::String, "required string value")
                 .switch("flag", "a flag for the signature", Some('f'))