[Turbo] Pass turbo_stream_listen topics to mercure if not defined

[Fan2Shrek]

Q	A
Bug fix?	yes
New feature?	no
Docs?	no
Issues	See below
License	MIT

Currently, the documentation says:

"If you're using a private hub, you can add { withCredentials: true } as the third argument to turbo_stream_listen() to authenticate with the hub."

However, this doesn’t actually work because the cookie is not created.
See: MercureExtension.php#L75

To make it work, we need to explicitly specify the topics again. The working call looks like this:
turbo_stream_listen("topics", "default", { subscribe: 'topics', withCredentials: true }

So I updated the implementation, adding the subscribe key if it's not already defined.

My mistake, sorry 😅

[Kocal]

Thanks @Fan2Shrek.