Logging site extension should use startup hooks (instead of additional deps)

The AzureAppServiceLoggingSiteExtension relies on the `DOTNET_ADDITIONAL_DEPS` feature in the host. This results in an error-prone process where we forget to add a new runtime store directory for older tfms when we update the repo's default tfm.

Given that 2.1 will go out of support soon, we will no longer need to support a version of .NET that doesn't have startup hooks. This will allow us to switch to a version agnostic implementation using startup hooks that will no longer require us to update the SiteExtension every time we add a tfm.