Allow configuration of diagnose endpoint
Allow configuration of diagnose endpoint

For the diagnose tests we need to send the diagnose JSON report to a
custom endpoint. This change introduces the APPSIGNAL_DIAGNOSE_ENDPOINT
environment variable for this purpose.

The APPSIGNAL_DIAGNOSE_ENDPOINT environment variable is not a variable
we'll document publicly, because it's only meant for testing purposes.

appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir