[BUG] Using NLog a required dependency isn't provded by NuGet

[johnjaylward]

ECS schema version (e.g. 1.4.0):
1.5.0

ECS .NET assembly version (e.g. 1.4.2):
1.5.0

Elasticsearch version (if applicable):
7.2+

.NET framework / OS:
windows dotnetcore 3.1

Description of the problem, including expected versus actual behavior:
When NLog starts up using a default configured EcsLayout, an exception is thrown that ElasticApmTraceId can not be found. This is because ElasticApmTraceId and ElasticApmTransactionId are defined in Package Elastic.Apm.NLog, however Elastic.Apm.NLog is not a required dependency of Elastic.CommonSchema.NLog through NuGet

Steps to reproduce:

1. Add NLog.Targets.ElasticSearch (alpha version, see above) and Elastic.CommonSchema.NLog 1.5.0 to your project.
2. Configure the project using the minimal options:

<extensions>
    <add assembly="NLog.Targets.ElasticSearch"/>
    <add assembly="Elastic.CommonSchema.NLog"/>
</extensions>
<targets>
   <target xsi:type="ElasticSearch" enableJsonLayout="true">
       <layout xsi:type="EcsLayout" />
   </target>
</targets>

3. Try to write a log message that is routed to the elastic target

exception is thrown:

System.TypeInitializationException
  HResult=0x80131534
  Message=The type initializer for 'Program' threw an exception.
  Source=Program
  StackTrace:
   at Program.<Main>d__7.MoveNext() in ....\Program.cs:line 34

  This exception was originally thrown at this call stack:
    [External Code]

Inner Exception 1:
NLogConfigurationException: Exception when parsing ....\bin\Debug\netcoreapp3.1\NLog.config. 

Inner Exception 2:
TargetInvocationException: Exception has been thrown by the target of an invocation.

Inner Exception 3:
ArgumentException: LayoutRenderer cannot be found: 'ElasticApmTraceId'

Expected Behavior:
Either the NuGet package should list Elastic.Apm.NLog as a dependency, or the Layout should be configured that it only uses items out of the Elastic.Apm.NLog package when it's available (possibly through reflection or a configuration option)