ConfigurationReader.FindConfigurationExtensionMethods relies on one specific implementation of ExtensionAttribute

[sajagi]

The method (ConfigurationReader.FindConfigurationExtensionMethods) should not rely on specific ExtensionAttribute type present in System.Runtime.dll, but rather on any type with matching name.

The attribute can be polyfilled in any assembly to enable extension method support.

In one (my) case this attribute is implemented in nunit.engine.core assembly which leads Serilog to not recognize extension methods when running tests using e.g. nunit3-console.

Happy to provide a PR.

[nblumhardt]

PR would be great, thanks. We should be able to squeeze this into #299.