Support for all custom value operations on plugin custom values (#12088)

# Description

Adds support for the following operations on plugin custom values, in
addition to `to_base_value` which was already present:

- `follow_path_int()`
- `follow_path_string()`
- `partial_cmp()`
- `operation()`
- `Drop` (notification, if opted into with
`CustomValue::notify_plugin_on_drop`)

There are additionally customizable methods within the `Plugin` and
`StreamingPlugin` traits for implementing these functions in a way that
requires access to the plugin state, as a registered handle model such
as might be used in a dataframes plugin would.

`Value::append` was also changed to handle custom values correctly.

# User-Facing Changes

- Signature of `CustomValue::follow_path_string` and
`CustomValue::follow_path_int` changed to give access to the span of the
custom value itself, useful for some errors.
- Plugins using custom values have to be recompiled because the engine
will try to do custom value operations that aren't supported
- Plugins can do more things 🎉 

# Tests + Formatting
Tests were added for all of the new custom values functionality.

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

# After Submitting
- [ ] Document protocol reference `CustomValueOp` variants:
  - [ ] `FollowPathInt`
  - [ ] `FollowPathString`
  - [ ] `PartialCmp`
  - [ ] `Operation`
  - [ ] `Dropped`
- [ ] Document `notify_on_drop` optional field in `PluginCustomValue`

tests/plugins/custom_values.rs

@@ -79,6 +79,57 @@ fn can_get_describe_plugin_custom_values() {
     assert_eq!(actual.out, "CoolCustomValue");
 }
 
+#[test]
+fn can_get_plugin_custom_value_int_cell_path() {
+    let actual = nu_with_plugins!(
+        cwd: "tests",
+        plugin: ("nu_plugin_custom_values"),
+        "(custom-value generate).0"
+    );
+
+    assert_eq!(actual.out, "abc");
+}
+
+#[test]
+fn can_get_plugin_custom_value_string_cell_path() {
+    let actual = nu_with_plugins!(
+        cwd: "tests",
+        plugin: ("nu_plugin_custom_values"),
+        "(custom-value generate).cool"
+    );
+
+    assert_eq!(actual.out, "abc");
+}
+
+#[test]
+fn can_sort_plugin_custom_values() {
+    let actual = nu_with_plugins!(
+        cwd: "tests",
+        plugin: ("nu_plugin_custom_values"),
+        "[(custom-value generate | custom-value update) (custom-value generate)] | sort | each { print } | ignore"
+    );
+
+    assert_eq!(
+        actual.out,
+        "I used to be a custom value! My data was (abc)\
+        I used to be a custom value! My data was (abcxyz)"
+    );
+}
+
+#[test]
+fn can_append_plugin_custom_values() {
+    let actual = nu_with_plugins!(
+        cwd: "tests",
+        plugin: ("nu_plugin_custom_values"),
+        "(custom-value generate) ++ (custom-value generate)"
+    );
+
+    assert_eq!(
+        actual.out,
+        "I used to be a custom value! My data was (abcabc)"
+    );
+}
+
 // There are currently no custom values defined by the engine that aren't hidden behind an extra
 // feature
 #[cfg(feature = "sqlite")]
@@ -116,3 +167,16 @@ fn fails_if_passing_custom_values_across_plugins() {
         .err
         .contains("the `inc` plugin does not support this kind of value"));
 }
+
+#[test]
+fn drop_check_custom_value_prints_message_on_drop() {
+    let actual = nu_with_plugins!(
+        cwd: "tests",
+        plugin: ("nu_plugin_custom_values"),
+        // We build an array with the value copied twice to verify that it only gets dropped once
+        "do { |v| [$v $v] } (custom-value drop-check 'Hello') | ignore"
+    );
+
+    assert_eq!(actual.err, "DropCheck was dropped: Hello\n");
+    assert!(actual.status.success());
+}