Allow plugins to report their own version and store it in the registry (#12883)

# Description

This allows plugins to report their version (and potentially other
metadata in the future). The version is shown in `plugin list` and in
`version`.

The metadata is stored in the registry file, and reflects whatever was
retrieved on `plugin add`, not necessarily the running binary. This can
help you to diagnose if there's some kind of mismatch with what you
expect. We could potentially use this functionality to show a warning or
error if a plugin being run does not have the same version as what was
in the cache file, suggesting `plugin add` be run again, but I haven't
done that at this point.

It is optional, and it requires the plugin author to make some code
changes if they want to provide it, since I can't automatically
determine the version of the calling crate or anything tricky like that
to do it.

Example:

```
> plugin list | select name version is_running pid
╭───┬────────────────┬─────────┬────────────┬─────╮
│ # │      name      │ version │ is_running │ pid │
├───┼────────────────┼─────────┼────────────┼─────┤
│ 0 │ example        │ 0.93.1  │ false      │     │
│ 1 │ gstat          │ 0.93.1  │ false      │     │
│ 2 │ inc            │ 0.93.1  │ false      │     │
│ 3 │ python_example │ 0.1.0   │ false      │     │
╰───┴────────────────┴─────────┴────────────┴─────╯
```

cc @maxim-uvarov (he asked for it)

# User-Facing Changes

- `plugin list` gets a `version` column
- `version` shows plugin versions when available
- plugin authors *should* add `fn metadata()` to their `impl Plugin`,
but don't have to

# Tests + Formatting

Tested the low level stuff and also the `plugin list` column.

# After Submitting
- [ ] update plugin guide docs
- [ ] update plugin protocol docs (`Metadata` call & response)
- [ ] update plugin template (`fn metadata()` should be easy)
- [ ] release notes

crates/nu_plugin_nu_example/nu_plugin_nu_example.nu

@@ -7,6 +7,7 @@
 # language without adding any extra dependencies to our tests.
 
 const NUSHELL_VERSION = "0.94.3"
+const PLUGIN_VERSION = "0.1.0" # bump if you change commands!
 
 def main [--stdio] {
   if ($stdio) {
@@ -229,6 +230,13 @@ def handle_input []: any -> nothing {
     }
     { Call: [$id, $plugin_call] } => {
       match $plugin_call {
+        "Metadata" => {
+          write_response $id {
+            Metadata: {
+              version: $PLUGIN_VERSION
+            }
+          }
+        }
         "Signature" => {
           write_response $id { Signature: $SIGNATURES }
         }