@overload type checking for overload signatures

I read through https://github.com/microsoft/pyright/issues/2806
Is there possibility of adding an opt in feature of type checking against overload signatures?
For default values, for example
```python
@overload
def f() -> str: ...

@overload
def f(a: int) -> int: ...

@overload
def f(a: str) -> str: ...

def f(a: int | str | None = None) -> int | str:
if a is None:
return "None"
return a
```
this `def f() -> str:` would be type checked the same as
```python
def f() -> str:
a = None
if a is None:
return "None"
return a
```

I know there are cases where this behavior doesn't work, but if this feature exists, I would just happily enable it and prohibit implementation that doesn't align with it.

Start by reading issue #2806 and the overload-signature behavior described in this issue. Locate Pyright's overload implementation checks and related tests, then define the opt-in configuration and edge-case behavior; done means the proposed default-value example and incompatible implementations are covered by tests.

Written by the indexing model from the issue text.