Support NestedMapping and MultiFieldsMapping in Release 2.0.8.1

[CCheSumo]

We haven't moved elastic-client to 6.x in metrics. Plan is to do that next quarter. Meanwhile, we will need to support nested mapping and multi-fields mapping. I am adding this change to this branch only atm. I will create a 2.0.9 branch for publishing after all functionalities has been included.

I am also adding a issue #164 to cherry-pick changes here to the latest master.

[codecov-io]

Codecov Report

Merging #163 into release-2.0.8 will increase coverage by 0.23%.
The diff coverage is 68.75%.

@@                 Coverage Diff                 @@
##             release-2.0.8     #163      +/-   ##
===================================================
+ Coverage            60.87%   61.11%   +0.23%     
  Complexity              88       88              
===================================================
  Files                   14       14              
  Lines                  961      972      +11     
  Branches               236      238       +2     
===================================================
+ Hits                   585      594       +9     
  Misses                 218      218              
- Partials               158      160       +2

Impacted Files	Coverage Δ	Complexity Δ
...ogic/elasticsearch/restlastic/dsl/MappingDsl.scala	49.03% <68.75%> (+3.87%)	0 <0> (ø)	⬇️

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[wojciechatsumo]

Great! I was about to introduce similar changes, nice that you got it before I even started! :)