🔨 refactor(custom.py): rename CustomChain to CustomAgentExecutor

🔨 refactor(base.py): add CustomAgentExecutor class and move CustomChain to base.py
🔨 refactor(custom_lists.py): update reference to CustomChain to CustomAgentExecutor
The CustomChain class has been renamed to CustomAgentExecutor to better reflect its purpose. The class has been moved to base.py and a new CustomAgentExecutor class has been added to custom.py. The reference to CustomChain in custom_lists.py has been updated to CustomAgentExecutor. These changes improve the semantics of the code and make it easier to understand the purpose of the classes.