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_python/nu_plugin_python_example.py

@@ -28,6 +28,7 @@ import json
 
 
 NUSHELL_VERSION = "0.94.3"
+PLUGIN_VERSION = "0.1.0" # bump if you change commands!
 
 
 def signatures():
@@ -228,7 +229,13 @@ def handle_input(input):
         exit(0)
     elif "Call" in input:
         [id, plugin_call] = input["Call"]
-        if plugin_call == "Signature":
+        if plugin_call == "Metadata":
+            write_response(id, {
+                "Metadata": {
+                    "version": PLUGIN_VERSION,
+                }
+            })
+        elif plugin_call == "Signature":
             write_response(id, signatures())
         elif "Run" in plugin_call:
             process_call(id, plugin_call["Run"])