Add env var to control sending telemetry to appinsights in functions worker

[lzchen]

Follow up to #1509

We have decided to separate out the context propagation and enabling azure monitor functionality. To do this, we will be using a separate env var PYTHON_APPLICATIONINSIGHTS_ENABLE_TELEMETRY, which will be for strictly sending telemetry to appinsights and as well enabling context propagation. Future configuration of the telemetry SDK will most likely have the word _APPLICATIONINSIGHTS_ inside it's name.

We treating Opentelemetry context propagation as a separate feature. This will be controlled as an opt-in like before with PYTHON_ENABLE_OPENTELEMETRY env var.