perf(schema) no deep copy on select on process auto fields

### Summary

It is inefficient to create deep copies of tables when e.g. looping through
database rows. In my testing with uploading some 64k routes with dbless
(which calls process auto fields twice), this can cut the time looping the
data by 1/4th. It also generates much less garbage.

I searched our code bases where we use "select" context, and could not
find anything that might break because of this.

kong/db/schema/init.lua

@@ -1609,11 +1609,15 @@ end
 -- valid values are: "insert", "update", "upsert", "select"
 -- @param nulls boolean: return nulls as explicit ngx.null values
 -- @return A new table, with the auto fields containing
--- appropriate updated values.
+-- appropriate updated values (except for "select" context
+-- it does it in place by modifying the data directly).
 function Schema:process_auto_fields(data, context, nulls, opts)
   local check_immutable_fields = false
 
-  data = tablex.deepcopy(data)
+  local is_select = context == "select"
+  if not is_select then
+    data = tablex.deepcopy(data)
+  end
 
   local shorthand_fields = self.shorthand_fields
   if shorthand_fields then
@@ -1664,8 +1668,6 @@ function Schema:process_auto_fields(data, context, nulls, opts)
   local now_s
   local now_ms
 
-  local is_select = context == "select"
-
   -- We don't want to resolve references on control planes
   -- and and admin api requests, admin api request could be
   -- detected with ngx.ctx.KONG_PHASE, but to limit context

spec/01-unit/01-db/01-schema/01-schema_spec.lua

@@ -2745,6 +2745,41 @@ describe("schema", function()
   end)
 
   describe("process_auto_fields", function()
+    for _, context in ipairs({ "insert", "update", "upsert"}) do
+      it('returns new table when called with "' .. context .. '" context', function()
+        local Test = Schema.new({
+          fields = {
+            { f = { type = "string", default = "test" } },
+          }
+        })
+
+        local original = {}
+        local data, err = Test:process_auto_fields(original, context)
+        assert.is_nil(err)
+        assert.not_equal(original, data)
+        if context == "update" then
+          assert.is_nil(data.f)
+        else
+          assert.equal("test", data.f)
+        end
+        assert.is_nil(original.f)
+      end)
+    end
+
+    it('modifies table in place when called with "select" context', function()
+      local Test = Schema.new({
+        fields = {
+          { f = { type = "string", default = "test" } },
+        }
+      })
+
+      local original = {}
+      local data, err = Test:process_auto_fields(original, "select")
+      assert.is_nil(err)
+      assert.equal(original, data)
+      assert.equal("test", data.f)
+      assert.equal("test", original.f)
+    end)
 
     it("produces ngx.null for non-required fields", function()
       local Test = Schema.new({