e_client: Introduce E_Client_Private

e_client: Introduce E_Client_Private

This commit introduces an E_Client_Private within the E_Client.

It's designed to store private data related to E_Client. This approach
prevents us from breaking ABI when adding or removing fields from the
E_Client structure. Instead, we can modify the E_Client_Private
structure as needed.

A pointer to E_Client_Private will be assigned to e_object's data and
accessed through e_object's data using e_object_data_{set,get}. So it's
crucial to ensure that no one overwrites it with different data using
e_object_data_set.

Change-Id: I4dff782d2d661f6eb0007253484640ebc9e3c2c4