Auto instrument OkHttp client with SentryOkHttpEventListener

[stefanosiano]

Description

Once the SentryOkHttpEventListener pr is merged, we could autoinstrument the OkHttp clients with it, other than the SentryOkHttpInterceptor.
The only thing to take care of is that OkHttp allows only one EventListener at a time, so we shouldn't add ours if the user already set one.
We can do this by either changing the default OkHttpClient.Builder constructor and inject our listener or by using a wrapper around it

[markushi]

Ideally we would use both, the existing interceptor (for adding trace headers) and the event listener.

[kahest]

@stefanosiano has this been resolved with #504 and #508?

[romtsn]

Closed by #504