Rename tables_clients.hpp and tables_clients.cpp to match the type name `Azure::Data::Tables::TableClient`

[ahsonkhan]

We currently have tables_clients.hpp (plural). Changing this after the GA would be considered breaking, so we'd want to fix it before https://github.com/Azure/azure-sdk-for-cpp/pull/6116

We have not made client plural in other SDKs (blob_client.hpp, key_client.hpp, etc.).

The type name is TableClient. https://github.com/Azure/azure-sdk-for-cpp/blob/0e699e4c7a39078daba7d902f9ee9b5b98b1b5cf/sdk/tables/azure-data-tables/inc/azure/data/tables/tables_clients.hpp#L167

The file name should match, either: table_client.hpp OR if we want to align with the package/namespace name tables_client.hpp

The same would apply to the source/cpp file, but that's less important since modifying that isn't considered breaking. https://github.com/Azure/azure-sdk-for-cpp/blob/0e699e4c7a39078daba7d902f9ee9b5b98b1b5cf/sdk/tables/azure-data-tables/src/tables_clients.cpp

cc @RickWinter