Create CallHandler annotation for Proxy methods

We should add a `@CallHandler` annotation for Proxy methods to process incoming calls in the way like this:
```java
@CallHandler
void onMethodCalled(List args);
```
```java
@CallHandler("methodName")
void onMethodCalled(List args);
```
```java
@CallHandler({"interfaceName", "methodName"})
void onMethodCalled(List args);
```
With this annotation the generated class for Proxy should be abstract, so that the User'll later implement handlers for methods with `@CallHandler` annotation.