Make low level client use System.text.json

[Mpdreamz]

re-targeting #4211 to master

This introduces an IElasticsearchSerializer implementation for System.Text.Json.

TODO:

• Remove LowLevelRequestResponseSerializer
• Move Utf8Json to Nest
• Make sure formatters the lowlevel client relies on are ported over
  • DynamicDictionaryFormatter

Long term plan is for Nest to move over to System.Text.Json as well but this will require significant investment in porting formatters over.

[Mpdreamz]

Kick this off again to see why clients-ci was failing.

[russcam]

rebased against master

[russcam]

canary packages are currently failing to be built for this because ILRepack cannot resolve the netstandard20 System.Text.Json assembly when running the "merge" for netstandard21

[2020-09-17T05:55:41.46+00:00][Elasticsearch.Net ][AssemblyDefinition     ] finished rewriting Elasticsearch.Net into D:\a\1\s\build\output\_packages\tmp\lib\netstandard2.1\Elasticsearch.Net8.dll
INFO: IL Repack - Version 2.1.0
INFO: Adding assembly for merge: D:\a\1\s\build\output\_packages\tmp\lib\netstandard2.1\Elasticsearch.Net8.dll
INFO: Processing references
INFO: Processing types
INFO: Merging <Module>
INFO: Processing types
INFO: Processing resources
INFO: Fixing references
Mono.Cecil.AssemblyResolutionException: Failed to resolve assembly: 'System.Text.Json, Version=4.0.1.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
   at Mono.Cecil.BaseAssemblyResolver.Resolve(AssemblyNameReference name, ReaderParameters parameters)
   at Mono.Cecil.DefaultAssemblyResolver.Resolve(AssemblyNameReference name, ReaderParameters pr)
   at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
   at Mono.Cecil.TypeReference.Resolve()
   at ILRepacking.ReferenceFixator.FixReferences(Collection`1 attributes)
   at ILRepacking.ReferenceFixator.FixReferences(TypeDefinition type)
   at ILRepacking.Steps.ReferencesFixStep.Perform()
   at ILRepacking.ILRepack.Repack()
   at ILRepacking.Application.Main(String[] args)

I've opened nullean/assembly-rewriter#7 to allow AssemblyRewriter to sign the rewritten versioned canary packages, which would allow ILRepack to be removed, since it is only being used to sign the assembly in the master and 7.x branches.

[Mpdreamz]

@russcam this is starting to look good to me! Some updated on this on my end

• clients-ci was silently failing open Fail the yaml runner if no folders are found #5024 to remedy this on all integration branches.
• Rebased this PR, clients-ci started failing because YamlMap is Dict<object, object> which System.Text.Json does not support. Added an additional converter for it.
• clients-ci is now failing on 1 test where the its asserting { value: 5.0 } where { value: 5 } is returned. We have some leniency if numbers are compared directly but we do not (and should not) recursively patch assertion values. This is related to the yaml test failures you identified in the rust client. We don't need to solve this on this PR IMO.
• Noticed some assertions in PostData.doc.cs were commented out (pretty sure I did that), uncommented them again and everthing was still green.

If you have time to:

• ignore that test for now
• run the docs generator to fix the stale docs check

I'm happy if we merge this to master 🎉

[Mpdreamz]

ResponseBuilder was asking the Utf8Json serializer to deserialize to DynamicDictionary.
I added a new System.Text.Json converter to handle this now.

This also updates DynamicValue to deal with the fact that deserializing to Dictionary<string, object> does not materialize object these remain JsonElement.

[Mpdreamz]

The failing integration tests relate to boost being available on some properties.

Merging this in!

[Mpdreamz]

Will follow up with a PR to:

• Remove LowLevelRequestResponseSerializer
• Move Utf8Json to Nest