Add config_file option

[felixbarny]

closes #701

• Implement code
• Add tests
• Update documentation
• Update CHANGELOG.md
• Update supported-technologies.asciidoc

[codecov-io]

Codecov Report

Merging #750 into master will decrease coverage by 0.47%.
The diff coverage is 77.08%.

@@             Coverage Diff              @@
##             master     #750      +/-   ##
============================================
- Coverage     64.89%   64.41%   -0.48%     
  Complexity       68       68              
============================================
  Files           210      210              
  Lines          8955     8821     -134     
  Branches       1157     1117      -40     
============================================
- Hits           5811     5682     -129     
- Misses         2784     2788       +4     
+ Partials        360      351       -9

Impacted Files	Coverage Δ	Complexity Δ
...apm/agent/impl/transaction/TraceContextHolder.java	53.19% <0%> (-5.15%)	0 <0> (ø)
...pentracing/impl/ApmSpanBuilderInstrumentation.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...lastic/apm/agent/impl/transaction/Transaction.java	84.42% <100%> (-0.28%)	0 <0> (ø)
...nt/configuration/ApmServerConfigurationSource.java	78.4% <100%> (-0.48%)	0 <0> (ø)
...lastic/apm/agent/logging/LoggingConfiguration.java	92.3% <100%> (+0.74%)	0 <0> (ø)	⬇️
...va/co/elastic/apm/agent/impl/ElasticApmTracer.java	74.05% <100%> (-1.7%)	0 <0> (ø)
...tic/apm/agent/configuration/CoreConfiguration.java	90.63% <72.72%> (-7.1%)	0 <0> (ø)
...astic/apm/agent/impl/transaction/TraceContext.java	89.28% <78.57%> (-1.14%)	0 <0> (ø)
...ic/apm/agent/slf4j/Slf4JMdcActivationListener.java	86.04% <83.33%> (-2.85%)	0 <0> (ø)
... and 8 more

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