HTTP client/server instrumentations should have an option to include headers as attirbutes

[tsloughter]

Each HTTP client/server instrumentation in this repo needs to add an option for including headers as attributes that follows the semconv:

https://github.com/open-telemetry/semantic-conventions/blob/main/docs/http/http-spans.md#http-request-and-response-headers

[albertored]

I can start with the cowboy and phoenix instrumentation