Document and blog post about versioned clients

[russcam]

Relates: #3913 (comment)

We have been releasing versioned clients to our CI package repository. A versioned client is IL-rewritten such that the assembly name and all namespaces are changed, allowing a versioned client to be referenced within a project in addition to an official release client package from nuget.

For example, with a CI nuget package source configured in a Nuget.config file

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="Appveyor CI" value="https://ci.appveyor.com/nuget/elasticsearch-net" />
  </packageSources>
</configuration>

A project can pull in the latest Nest.v6 package from CI, in addition to Nest package version 7.0.0 from Nuget. For example,

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>netcoreapp2.2</TargetFramework>
    </PropertyGroup>
    <ItemGroup>
      <PackageReference Include="NEST" Version="7.0.0" />
      <PackageReference Include="NEST.v6" Version="6.9.0-ci20190620T132817" />
    </ItemGroup>
</Project>

Then, both can be used in the project side by side

public static void Main()
{
    // 6.x client from Nest.v6 package
    var settings6x = new Nest6.ConnectionSettings(new Elasticsearch.Net6.SingleNodeConnectionPool(uri))
        .DefaultIndex("posts");

    var client6x = new Nest6.ElasticClient(settings6x);

    // 7.x client from Nest 7.0.0 package
    var settings = new ConnectionSettings(new SingleNodeConnectionPool(uri))
        .DefaultIndex("posts");

    var client = new ElasticClient(settings);
}

and custom logic can be implemented to determine which client to use.

The intention with providing these is to help in the short-term in upgrading the client to the next major version, at the same time as upgrading Elasticsearch.

Documentation and a blog post should be provided to highlight.

[codebrain]

Will look at writing after 7.3.0 release.

Look into including in the README.MD and outlining support.

[codebrain]

Closing as blog post was published with an example of how to do this:
https://www.elastic.co/blog/nest-and-elasticsearch-net-upgrading-your-codebase

[benbenwilde]

the feed in the blog post doesn't have v6 so i thought i was out of luck. but i found v6 in the appveyor feed mentioned here.