DropdownMenu should manage its own internal FocusNode when focusNode is not provided

## Proposal
`DropdownMenu` should adopt the `_effectiveFocusNode` pattern (already used in `TextField` and other widgets) so that it always has a `FocusNode` available internally, even when the `focusNode` parameter is not provided.

## Motivation
This change is needed to support the `resetOnBlur` feature proposed in #180584. The `resetOnBlur` implementation requires listening to focus changes, but currently `DropdownMenu` only has access to a `FocusNode` when one is explicitly provided via the `focusNode` parameter. This forces developers to always provide a `focusNode` to use `resetOnBlur`, which is an unreasonable API limitation.

By managing an internal `FocusNode` when one is not provided, `DropdownMenu` will always have a focus node to listen to, removing this restriction.

Start with the DropdownMenu implementation and compare its focus handling with the _effectiveFocusNode pattern already used in TextField and other widgets. The change is done when DropdownMenu has an internal FocusNode whenever focusNode is omitted, while still supporting a provided node for the resetOnBlur work described in #180584; check linked pull request #183684 before starting.

Written by the indexing model from the issue text.