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`

crates/nu-command/src/database/values/sqlite.rs

@@ -372,19 +372,29 @@ impl CustomValue for SQLiteDatabase {
         self
     }
 
-    fn follow_path_int(&self, _count: usize, span: Span) -> Result<Value, ShellError> {
+    fn follow_path_int(
+        &self,
+        _self_span: Span,
+        _index: usize,
+        path_span: Span,
+    ) -> Result<Value, ShellError> {
         // In theory we could support this, but tables don't have an especially well-defined order
-        Err(ShellError::IncompatiblePathAccess { type_name: "SQLite databases do not support integer-indexed access. Try specifying a table name instead".into(), span })
+        Err(ShellError::IncompatiblePathAccess { type_name: "SQLite databases do not support integer-indexed access. Try specifying a table name instead".into(), span: path_span })
     }
 
-    fn follow_path_string(&self, _column_name: String, span: Span) -> Result<Value, ShellError> {
-        let db = open_sqlite_db(&self.path, span)?;
+    fn follow_path_string(
+        &self,
+        _self_span: Span,
+        _column_name: String,
+        path_span: Span,
+    ) -> Result<Value, ShellError> {
+        let db = open_sqlite_db(&self.path, path_span)?;
 
-        read_single_table(db, _column_name, span, self.ctrlc.clone()).map_err(|e| {
+        read_single_table(db, _column_name, path_span, self.ctrlc.clone()).map_err(|e| {
             ShellError::GenericError {
                 error: "Failed to read from SQLite database".into(),
                 msg: e.to_string(),
-                span: Some(span),
+                span: Some(path_span),
                 help: None,
                 inner: vec![],
             }