Use Record's public API in a bunch of places (#10927)

# Description
Since #10841 the goal is to remove the implementation details of
`Record` outside of core operations.

To this end use Record iterators and map-like accessors in a bunch of
places. In this PR I try to collect the boring cases where I don't
expect any dramatic performance impacts or don't have doubts about the
correctness afterwards

- Use checked record construction in `nu_plugin_example`
- Use `Record::into_iter` in `columns`
- Use `Record` iterators in `headers` cmd
- Use explicit record iterators in `split-by`
- Use `Record::into_iter` in variable completions
- Use `Record::values` iterator in `into sqlite`
- Use `Record::iter_mut` for-loop in `default`
- Change `nu_engine::nonexistent_column` to use iterator
- Use `Record::columns` iter in `nu-cmd-base`
- Use `Record::get_index` in `nu-command/network/http`
- Use `Record.insert()` in `merge`
- Refactor `move` to use encapsulated record API
- Use `Record.insert()` in `explore`
- Use proper `Record` API in `explore`
- Remove defensiveness around record in `explore`
- Use encapsulated record API in more `nu-command`s

# User-Facing Changes
None intentional

# Tests + Formatting
(-)

crates/nu_plugin_example/src/example.rs

@@ -67,13 +67,7 @@ impl Example {
                     .map(|v| Value::int(v * i, call.head))
                     .collect::<Vec<Value>>();
 
-                Value::record(
-                    Record {
-                        cols: cols.clone(),
-                        vals,
-                    },
-                    call.head,
-                )
+                Value::record(Record::from_raw_cols_vals(cols.clone(), vals), call.head)
             })
             .collect::<Vec<Value>>();