add propagation interface#510

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asanluis wants to merge 8 commits intoopenzipkin:masterfrom

asanluis:496-Add_propagation_interface

Commits on Jul 15, 2020

start propagation interface
asanluis
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Commits on Jul 27, 2020

start propagation interface
asanluis
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Commits on Aug 5, 2020

Commits on Sep 21, 2020

Commits on Dec 2, 2020

add Extractor and Injector implementation
asanluis
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