### 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_
from threading import RLock
from typing import Any, Generic, Type, TypeVar, Union
from localstack.utils.aws.aws_stack import get_valid_regions_for_service
LOCAL_ATTR_PREFIX = "attr_"
BaseStoreType = TypeVar("BaseStoreType")
#
#
class LocalAttribute:
    """
    Descriptor protocol for marking store attributes as local to a region.
    """
    def __init__(self, default: Union[Callable, int, float, str, bool, None]):
        """
        :param default: Default value assigned to the local attribute. Must be a scalar
            or a callable.
        """
        self.default = default
    def __set_name__(self, owner, name):
        self.name = LOCAL_ATTR_PREFIX + name
    def __get__(self, obj: BaseStoreType, objtype=None) -> Any:
        if not hasattr(obj, self.name):
            if isinstance(self.default, Callable):
                value = self.default()
            else:
                value = self.default
            setattr(obj, self.name, value)
        return getattr(obj, self.name)
    def __set__(self, obj: BaseStoreType, value: Any):
        setattr(obj, self.name, value)
class CrossRegionAttribute: