perf: better scalability of get_columns (#15780)

# Description

Use hashset for existence checking.
Still needs a vector collection to keep the column order for tables.

# User-Facing Changes

Should be None

crates/nu-engine/src/column.rs

@@ -2,14 +2,15 @@ use nu_protocol::Value;
 use std::collections::HashSet;
 
 pub fn get_columns(input: &[Value]) -> Vec<String> {
-    let mut columns = vec![];
+    let mut column_set = HashSet::new();
+    let mut columns = Vec::new();
     for item in input {
         let Value::Record { val, .. } = item else {
             return vec![];
         };
 
         for col in val.columns() {
-            if !columns.contains(col) {
+            if column_set.insert(col) {
                 columns.push(col.to_string());
             }
         }