refactor Value::follow_cell_path to reduce clones and return Cow (#15640)

# Description
While working on something else, I noticed that
`Value::follow_cell_path` receives `self`.

While it would be ideal for the signature to be `(&'a self, cell_path)
-> &'a Value`, that's not possible because:
1. Selecting a row from a list and field from a record can be done with
a reference but selecting a column from a table requires creating a new
list.
2. `Value::Custom` returns new `Value`s when indexed.

So the signature becomes `(&'a self, cell_path) -> Cow<'a, Value>`.

Another complication that arises is, once a new `Value` is created, and
it is further indexed, the `current` variable
1. can't be `&'a Value`, as the lifetime requirement means it can't
refer to local variables
2. _shouldn't_ be `Cow<'a, Value>`, as once it becomes an owned value,
it can't be borrowed ever again, as `current` is derived from its
previous value in further iterations. So once it's owned, it can't be
indexed by reference, leading to more clones

We need `current` to have _two_ possible lifetimes
1. `'out`: references derived from `&self`
2. `'local`: references derived from an owned value stored in a local
variable

```rust
enum MultiLife<'out, 'local, T>
where
    'out: 'local,
    T: ?Sized,
{
    Out(&'out T),
    Local(&'local T),
}
```
With `current: MultiLife<'out, '_, Value>`, we can traverse values with
minimal clones, and we can transform it to `Cow<'out, Value>` easily
(`MultiLife::Out -> Cow::Borrowed, MultiLife::Local -> Cow::Owned`) to
return it

# User-Facing Changes

# Tests + Formatting

# After Submitting

---------

Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>

crates/nu-cmd-extra/src/extra/strings/format/command.rs

@@ -253,12 +253,11 @@ fn format_record(
                         optional: false,
                     })
                     .collect();
-                match data_as_value.clone().follow_cell_path(&path_members, false) {
-                    Ok(value_at_column) => {
-                        output.push_str(value_at_column.to_expanded_string(", ", config).as_str())
-                    }
-                    Err(se) => return Err(se),
-                }
+
+                let expanded_string = data_as_value
+                    .follow_cell_path(&path_members, false)?
+                    .to_expanded_string(", ", config);
+                output.push_str(expanded_string.as_str())
             }
         }
     }