Proposal: Support Class-Based Syntax for Python Entities

Currently, Entities in Python (and JavaScript) only support function-based syntax, where operations are delineated by a cascade of if-statements. In C#, a class-based syntax is supported, which allows individual operations to be split into methods belonging to a larger class that represents the Entity itself.

Implementing this alternative syntax should not be too difficult, it's mostly just a layer of indirection. Making matters simpler, we already have a community-driven example of how to do this for TypeScript here: https://scale-tone.github.io/2021/03/15/durable-mvc .

As an example for the syntax, consider the following entity function using the current implementation syntax

```Python
def entity_function(context: df.DurableEntityContext):
current_value = context.get_state(lambda: 0)
operation = context.operation_name
if operation == "add":
amount = context.get_input()
current_value += amount
elif operation == "reset":
current_value = 0
elif operation == "get":
pass

context.set_state(current_value)
context.set_result(current_value)
```

It's class-based syntax alternative would look as follows:

```Python
class Counter:
def __init__(self): # this corresponds to state initializer, or the `lambda: 0` in the function-based syntax
self.entity_state= 0

def add(self, amount):
self.entity_state+= amount
return self.entity_state
def get(self):
return self.entity_state
def reset(self):
return self.entity_state= 0
```

Finally, it makes sense to me that the `return` statement would set the value for `context.set_result`. A more difficult question is to know what value to save for `set_state`. One easy solution is to require the state to be under the name `self.entity_state` but perhaps that's too restrictive. What do others think?

The issue does not name any files or tests. Start by reviewing the existing function-based entity handling around DurableEntityContext and the linked TypeScript example, then determine how class methods, initialization, returned results, and persisted state should map to the current entity model. Done requires an agreed syntax and state-handling design before implementation.

Written by the indexing model from the issue text.