Split com.google.cloud.trace:sink into sink + consumer

[bogdandrutu]

Consumer-API (exporter, reporter):

• TraceConsumer interface
• TraceConsumer implementations
• Minimal dependencies (almost nothing - proto-google-cloud-trace-v1)

Stackdriver-Consumer (depends on google-cloud-java + consumer-api)

• gRPC/HTTP implementations of these.

Sink will have the sinkv1 + producers.

[bogdandrutu]

@saturnism