Allow to construct and send SpanData to all configured exporters

[SergeyKanzhelev]

There are scenarios when spans may need to be reported after actual work was already completed. The proposal was to allow this scenario by requiring to construct SpanData explicitly and send it to the export component.

In order to implement it - export component need to expose the Export method that accepts SpanData. So implementation will not be required to get access to the handlers.

For C#: https://github.com/census-instrumentation/opencensus-csharp/blob/2e9f34e452b04831ce65b5894748f8045fef77da/src/OpenCensus.Abstractions/Trace/Export/ISpanExporter.cs#L35-L43