Redis output caching garbage collection routine should create an Activity to represent its work

If you create a new .NET Aspire application using the starter template, then run the app, hit the website, and then leave it running for at least 5 minutes, you'll see the following traces appear in the dashboard:

These are 4 root-level spans that represent the Redis operations performed by the [Redis output caching provider's garbage collection routine](https://github.com/dotnet/aspnetcore/blob/f545e80531c098e7c1611eca861b0f080fa14a3c/src/Middleware/Microsoft.AspNetCore.OutputCaching.StackExchangeRedis/src/RedisOutputCacheStore.cs#L126).

Since these are part of the same logical operation, the provider should create an `Activity` to represent the garbage collection iteration so that these spans created by the Redis client are collected as children of a parent span representing the garbage collection operation.