Fix asp.net sample to use DI

[jakejscott]

Fixes DI for the asp.net sample

1. Closes Fix aspnet DI sample #47

2. How was this tested:
   Manually by running the web and worker and hitting http://localhost:5000

3. Any docs updates needed?
   N/A

[cretz]

This was already using DI. Based on https://stackoverflow.com/questions/43240405/async-provider-in-net-core-di and others I've seen, they suggest that async be deferred until needed hence why I put the task in the container (though in retrospect maybe it should have been a lazy task).

I wonder if TemporalClientAdapter or TemporalClientProvider or whatever is something we should add to Temporalio.Extensions.Hosting. But it'd just be a lazy wrapper.