remove legacy TelemetryConfiguration.Active from AspNetCore SDK

[TimothyMothra]

Currently, AspNetCore depends on TelemetryConfiguration.Active and would copy all configured options to this singleton.
Once this dependency is removed we will no longer have a need for this singleton.

(For further reading.. this dependency is due to the Heartbeat feature #1954).

For backwards compatibility, we will introduce a new setting: ApplicationInsightsServiceOptions.EnableActiveTelemetryConfigurationSetup.
This will once again copy the TelemetryConfiguration configured via DI to the TelemetryConfiguration.Active singleton.