Separate subclient from service client

We don't want to check subclient for AZC0004. According to https://azure.github.io/azure-sdk/dotnet_introduction.html#dotnet-subclient-factory-methods-suffix, we have different patterns to create get subclient methods. We should find out a solution to separate subclient from service client, details in https://github.com/Azure/azure-sdk-tools/pull/6496#discussion_r1267354044

Start by reading the .NET subclient factory-method guidance linked in the issue and the referenced azure-sdk-tools pull-request discussion. Determine how AZC0004 currently distinguishes subclients from service clients, then define the separation behavior and verify it against the patterns described in those references.

Written by the indexing model from the issue text.