Add tracer_provider exporters and span_processors

[jack-berg]

An alternative to #16 which breaks out a separate tracer_provider.exporters block whose keys can be referenced in span processor configuration.

This is related to the suggestion from @srikanthccv here.

[codeboten]

This makes sense to me, the only question would be whether exporters should be a top level configuration group, but we can always move forward with this and revisit later if we find that commonalities across signals make sense to pull up.