A spawn trait for Commands and ChildBuilder

## What problem does this solve or what need does it fill?

Both `Commands` and `ChildBuilder` share the methods `spawn` and `spawn_bundle`.

In my project I create a `UiExt` that add methods to spawn some ui related structure of entity (e.g. for scrolling list) that look like this :

```rust
pub trait UiExt<'w, 's> {
fn spawn<'a>(&'a mut self) -> EntityCommands<'w, 's, 'a>;

fn spawn_scroll_list<'a>(
&'a mut self,
style: Style,
spawn_items: impl FnOnce(&mut ChildBuilder),
) -> EntityCommands<'w, 's, 'a> {
let mut entity_commands = self.spawn();
scroll_list::build_scroll_list(&mut entity_commands, style, spawn_items);
entity_commands
}
}

impl<'w, 's> UiExt<'w, 's> for Commands<'w, 's> {
fn spawn<'a>(&'a mut self) -> EntityCommands<'w, 's, 'a> {
self.spawn()
}
}

impl<'w, 's, 'a> UiExt<'w, 's> for ChildBuilder<'w, 's, 'a> {
fn spawn<'b>(&'b mut self) -> EntityCommands<'w, 's, 'b> {
self.spawn()
}
}
```

The methods are the same whatever if self is `Commands` or `ChildBuilder` as long as I can get a `EntityCommands`.
But because `spawn` is not implemented via a trait I cannot simply use a blanket implementation.

## What solution would you like?

Expose `spawn` via a trait for `Commands` and `ChildBuilder`.

Allowing blanket implementation:

```rust
impl<'w, 's, T> UiExt<'w, 's> for T
where
T : SpawnEntityCommands<'w, 's>
{
fn spawn_scroll_list<'a>(
&'a mut self,
style: Style,
spawn_items: impl FnOnce(&mut ChildBuilder),
) -> EntityCommands<'w, 's, 'a> {
let mut entity_commands = self.spawn();
scroll_list::build_scroll_list(&mut entity_commands, style, spawn_items);
entity_commands
}
}
```

## What alternative(s) have you considered?

Add a `spawn` method in my `UiExt` trait and manually implement it for `Commands` and `ChildBuilder`.

Start by locating the Commands::spawn and ChildBuilder::spawn entry points and compare their shared return type and lifetimes. Determine where a common spawn trait belongs and how existing API tests cover these methods. Done means both types expose the trait-based spawn behavior and the motivating blanket UiExt implementation can use it without duplicating spawn methods.

Written by the indexing model from the issue text.