[FEATURE] Unify NLog and Serilog ECS representations

[codebrain]

Consider moving "common" code used by both Serilog and NLog into the Elastic.CommonSchema project, so that they are in sync. It will be interesting to see if this is inherently possible in all cases as the Elastic.CommonSchema project should not contain references to any logging libraries.

For example Serilog CatchErrors includes StackTrace for inner-exceptions But NLog CatchError does not (At the same time you can optimize Serilog to not allocate unnecessary Exception-List)