Implement heartbeats for Elixir

[unflxw]

Part of https://github.com/appsignal/appsignal-ruby/issues/1054.

See also test setup: https://github.com/appsignal/test-setups/pull/192

Remove use Appsignal.Config macro

I see no reason why this needs to be a macro. This could just be a method. So it is a method now.

Implement Transmitter.transmit method

Implement a transmit method, similar to that in the Ruby integration, that sends a POST request with the API key, environment, application name and hostname as query parameters.

Use it where requests with these parameters are sent, that is, for the push API key validation request, and for the diagnose report request.

Fix a bug where config[:environment] (a non-existent configuration key) was accessed instead of config[:env].

Implement heartbeats

Implement the heartbeat functionality using the transmitter. The implementation is functionally identical to the one for the Ruby integration: https://github.com/appsignal/appsignal-ruby/pull/1057

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[unflxw]

Moved this PR to Blocked. It is ready to merge any time, ideally shortly before the user-facing feature in the product is available.