Adds possibility to set headers to zipkin exporter

[obecny]

Which problem is this PR solving?

• Fixes Can't Set Authorization Headers for Zipkin Exporter #1200

Short description of the changes

• It adds possibility to set headers to zipkin exporter

[codecov]

Codecov Report

Merging #1202 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1202   +/-   ##
=======================================
  Coverage   92.32%   92.32%           
=======================================
  Files         122      122           
  Lines        3533     3533           
  Branches      714      714           
=======================================
  Hits         3262     3262           
  Misses        271      271           

Impacted Files	Coverage Δ
...ackages/opentelemetry-exporter-zipkin/src/types.ts	100.00% <ø> (ø)
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	98.46% <ø> (ø)

[mwear]

LGTM

[mayurkale22]

It would be nice if you can update the Usage section in Readme (https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-exporter-zipkin/README.md) to highlight this new option, otherwise there is no easy way for new users to know about this option.

[mayurkale22]

/cc @astorm