Consistently handle common methods in callback handlers

[ttddyy]

Currently, common method invocations (toString, hashCode, unwrap, etc) are handled in individual callback handler and their parent class. As a result, they are inconsistently handled.

Instead, they should be handled in one place, guaranteeing consistent behavior across all proxies.

In addition, it needs to address some missing handling logic:

• equals to consider proxy object
• unwrap to consider the target class argument
• hashCode handling