Zenject/Extenject support for SpacetimeDBNetworkManager

[FluentCoding]

It would be pretty cool if the SDK could ship a separate Zenject-/Extenject compatible network manager implementation (probably locked behind a symbol like ZENJECT_DEFINED) which would allow for a more natural integration of the SDK into an already zenject-based unity project. The new implementation would be straightforward, removing the singleton logic entirely, replacing Update/OnDestroy with ITickable and IDisposable and swapping instance access here: 1, 2. If there's enough interest, I would be down to write a pull request for this.