Add proxy support.

[BrandonTheBuilder]

This PR adds support for sending data to Newrelic via a proxy.

Closes: #17

[codecov-commenter]

Codecov Report

Merging #30 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #30   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          302       322   +20     
  Branches        23        26    +3     
=========================================
+ Hits           302       322   +20     

Impacted Files	Coverage Δ
src/newrelic_telemetry_sdk/client.py	100.00% <100.00%> (ø)

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