Add support for API Key auth

[axw]

Add support for API Key auth. API Key auth can be configured by specifying a base64-encoded API Key via the environment variable ELASTIC_APM_API_KEY.

We also add support for running the Gherkin feature tests defined in https://github.com/elastic/apm/tree/master/tests/agents/gherkin-specs

Closes #694

[codecov-io]

Codecov Report

Merging #698 into master will decrease coverage by 0.88%.
The diff coverage is 81.14%.

@@            Coverage Diff            @@
##           master    #698      +/-   ##
=========================================
- Coverage   85.59%   84.7%   -0.89%     
=========================================
  Files         125     127       +2     
  Lines        8516    8260     -256     
=========================================
- Hits         7289    6997     -292     
- Misses        865     884      +19     
- Partials      362     379      +17

Impacted Files	Coverage Δ
transport/http.go	85.84% <77.77%> (-0.91%)	⬇️
internal/apmgodog/formatter.go	79.03% <79.03%> (ø)
internal/apmgodog/suitecontext.go	84.31% <84.31%> (ø)
config.go	76.88% <0%> (-10.23%)	⬇️
tracer.go	88.3% <0%> (-3.45%)	⬇️
breakdown.go	95.2% <0%> (-1.32%)	⬇️
transaction.go	93.33% <0%> (-1.3%)	⬇️
model/marshal_fastjson.go	66.94% <0%> (-0.28%)	⬇️
... and 2 more

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[mdelapenya]

Hey @axw, this is great!!! Very happy you added Godog in the agent.