Fix client factory extension method validation issues

[JamesNK]

Fixes #2158

• Extension methods now work when AddGrpcClient is called without configuration
• Extension methods now report the failing method name correctly (I made it explicit instead of an attribute to avoid someone unknowingly accidentally regressing the error name)

[JamesNK]

FYI @jviau

You can temporarily work around the issue by using an AddGrpcClient overload that includes a configuration parameter and provide an empty one.

[jviau]

Thanks for the quick fix! And yes, I am using the .AddGrpcClient(_ => { }) for now.

[mgravell]

approved with one optional suggestion

edit: I just noticed the "(I made it explicit instead of an attribute to avoid someone unknowingly accidentally regressing the error name)" (sorry, words are hard!) - noted, so: you considered it, dismissed it, and gave a reason; fine