Add ability to adjust spans before reporting them

[marcingrzejszczak]

Adding a functional interface to adjust the Zipkin's SpanBuilder before the Span is built and reported could allow to highly customize the built span according to one's needs without adding anything to the architecture.

For example for this issue #500 (comment) one could apply a customizer to verify the the tags to retrieve the URL and then rename the span accordingly.

Similar to this: https://github.com/openzipkin/zipkin-sparkstreaming#adjuster

cc @adriancole @ouaibsky @s-spindler

[marcingrzejszczak]

Done here #567