Support older ES client versions

[eyalkoren]

The new module is called 5.6 because it depends on the 5.6 version. I chose this version because it is the first ES client version that has both low-level and high-level client APIs and I wanted to test against those. However, as far as I can tell, this code supports all versions since 5.0.2.
Also, it should be safe for all versions continuously, meaning that for each version from 5.0.2 and to current only a single instrumentation is applied and only the right one.

NOTE: I tested everything with current ES version, assuming that as long as the client works well on it, it doesn't matter. Let me know if you think this is a wrong assumption

fixes #267

[codecov-io]

Codecov Report

Merging #268 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #268   +/-   ##
=========================================
  Coverage     73.29%   73.29%           
  Complexity     1158     1158           
=========================================
  Files           124      124           
  Lines          4322     4322           
  Branches        431      431           
=========================================
  Hits           3168     3168           
  Misses          947      947           
  Partials        207      207

Impacted Files	Coverage Δ	Complexity Δ
...lastic/apm/plugin/api/ApiScopeInstrumentation.java	71.42% <ø> (ø)	5 <0> (ø)	⬇️
...a/co/elastic/apm/report/ReporterConfiguration.java	100% <0%> (ø)	11% <0%> (ø)	⬇️

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