Support for alternative HTTP Clients

## What feature would you like to see?

`CctTransportBackend.doSend()` is hardcoded to use HttpURLConnection.

https://github.com/firebase/firebase-android-sdk/blob/master/transport/transport-backend-cct/src/main/java/com/google/android/datatransport/cct/CctTransportBackend.java

Ideally this would be easily configurable to share the HTTP Client with other parts of the app, such as image loaders, APIs (retrofit).

This is possible with `URL.setURLStreamHandlerFactory` to either Cronet or OkHttp (via [ObsoleteUrlFactory](https://gist.github.com/swankjesse/dd91c0a8854e1559b00f5fc9c7bfae70))

While it seems possible to replace it via TransportBackendDiscovery in the manifest, it would duplicate a lot of code from CctTransportBackend.

## How would you use it?

For Wear Apps, using OkHttp provides a centralised place to restrict requests when it isn't appropriate, such as avoiding using LTE and logging network traffic. OkHttp has event listeners and interceptors that allow such things.

Start with transport/transport-backend-cct/src/main/java/com/google/android/datatransport/cct/CctTransportBackend.java, especially doSend(), and compare its behavior with TransportBackendDiscovery. Determine how an application could provide or select an HTTP client without duplicating CctTransportBackend; done means the transport backend supports the requested configuration while preserving its existing behavior and the Wear app use case.

Written by the indexing model from the issue text.