This commit adds support for creating `resource()`s with streaming response
data. A streaming resource is defined by a `stream` option instead of a
`loader`, with `stream` being a function returning
`Promise<Signal<{value: T}|{error: unknown}>>`. Once the streaming loader
resolves to a `Signal`, it can continue to update that signal over time, and
the values (or errors) will be delivered to via the resource's state.

`rxResource()` is updated to leverage this new functionality to handle
multiple responses from the underlying Observable.

PR Close angular#59573

With the changes in angular#59573, `resource` can now define a `stream` rather than a `loader`.
In the same PR, `rxResource` was updated to leverage this new functionality to handle multiple responses from the underlying observable,
rather than just the first one as it was previously.
This commit renames the `loader` option of `rxResource` into `stream` to be better aligned with its new behavior.

The previous version is temporarily kept and marked as deprecated to help migrating the current usage.

Before
```
usersResource = rxResource({
  request: () => ...,
  loader: ({ request }) => ...
});
```

After
```
usersResource = rxResource({
  request: () => ...,
  stream: ({ request }) => ...
});
```