Add context casting functionality

Please consider functionality that allows to validate if context A can be cast into context B at model definition time even when there is no direct inheritance between A and B. Consider the following example:

from ccflow import ContextBase

class A(ContextBase):
   foo: str = "x"
class B(ContextBase):
   foo: str = "x"

B.model_validate(A())

This is fully supported use case, but the limitation is that it requires an instance of A(). It would be helpful if we could do something like:

B.context_validate(A)

which returns True if context A can be cast to context B or False otherwise.

Start at ContextBase and the existing model_validate path to understand how one context is cast into another without an instance. Define the expected class-level behavior for compatible and incompatible contexts, then verify that context_validate(A) returns the corresponding boolean for the example and its failure cases.