Allow Lobby components to use a custom instance of StreamVideo

[ptrbrynt]

🎯 Goal

This allows the StreamLobbyView and StreamLobbyVideo to use a custom instance of StreamVideo.

This ensures that apps which initialize Stream using StreamVideo.create can still use these components.

Resolves one of the suggestions in #741

🛠 Implementation details

Add an optional streamVideo parameters to the constructors of StreamLobbyView and StreamLobbyVideo. If this parameter is omitted, the widgets will fall back to StreamVideo.instance.

🎨 UI Changes

Add relevant screenshots

Before	After
img	img

Add relevant videos

Before	After

🧪 Testing

Pass in a non-singleton instance of StreamVideo to the two Lobby widgets and verify that they use the given instance. Also verify that the behaviour remains the same otherwise.

☑️Contributor Checklist

General

• Assigned a person / code owner group (required)
• Thread with the PR link started in a respective Slack channel (#flutter-team) (required)
• PR is linked to the GitHub issue it resolves

☑️Reviewer Checklist

• Sample runs & works
• UI Changes correct (before & after images)
• Bugs validated (bugfixes)
• New feature tested and works
• All code we touched has new or updated Documentation

[Brazol]

Looks good, thanks for your contribution @ptrbrynt!