### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

I prefer not to use the real regions when doing tests or local stuff to make sure that if it accidentally picks up real credentials, it won't actually create or interact with resources in AWS. This used to work find in localstack - I even dug through the code to find that there is an option in the `RegionStore` to disable validation of the region string. At some point, you removed any way to disable that validation though, which means it just fails all requests using a custom region.

### 🧑‍💻 Implementation

There's already a flag that is passed to the account and region store constructors to disable validation. Seems like an environment variable or configuration flag to disable validation could be propagated.

### Anything else?

_No response_
"""
    Descriptor protocol for marking store attributes as shared across all regions.
    """
    def __init__(self, default: Union[Callable, int, float, str, bool, None]):
        """
        :param default: The default value assigned to the cross-region attribute.
            This must be a scalar or a callable.
        """
        self.default = default
    def __set_name__(self, owner, name):
        self.name = name
    def __get__(self, obj: BaseStoreType, objtype=None) -> Any:
        self._check_region_store_association(obj)
        if self.name not in obj._global.keys():
            if isinstance(self.default, Callable):
                obj._global[self.name] = self.default()
            else:
                obj._global[self.name] = self.default
        return obj._global[self.name]
    def __set__(self, obj: BaseStoreType, value: Any):
        self._check_region_store_association(obj)
        obj._global[self.name] = value
    def _check_region_store_association(self, obj):
        if not hasattr(obj, "_global"):
            
            raise AttributeError(
                "Could not resolve cross-region attribute because there is no associated RegionBundle"
            )
class CrossAccountAttribute: