Support IStartupFilter

[dazinator]

IStartupFilter's are described here: https://andrewlock.net/exploring-istartupfilter-in-asp-net-core/

1. Should per tenant middleware pipeline support running IStartupFilters registered at tenant level?

If so, these could be used to perform tenant startup tasks like:

1. Initialise the tenants virtual file system.
2. Initialise / build the tenants configuration (I currently allow this to be lazily built when Task is first resolved anywhere, but this change would mean it would be always built when tenant first initialised)

[dazinator]

Note: The azure signal r sdk relies on IStartupFilter to function: https://github.com/Azure/azure-signalr/blob/8a52c52c3a236abbbbbab02c8e614130a365265d/src/Microsoft.Azure.SignalR/Startup/AzureSignalRStartupFilter.cs

So supporting this is going to be needed in order to enable azure signal-r to work at a per tenant level.