Add option to change agent from Auth() init

[mkmer]

Description:

A little forward thinking (maybe) - allow calling app to set agent and app_build as optional keys.
I have a feeling we will need to change this more often than not.

Related issue (if applicable): fixes #

Checklist:

• Local tests with tox run successfully PR cannot be meged unless tests pass
• Changes tested locally to ensure platform still works as intended
• Tests added to verify new code works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5d83708) 99.86% compared to head (ec8f682) 99.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #860   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files           8        8           
  Lines        1477     1477           
=======================================
  Hits         1475     1475           
  Misses          2        2           

Flag	Coverage Δ
unittests	99.86% <100.00%> (ø)

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[fronzbot]

Yeah makes sense to me