### 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_
_provider_config: Dict[str, str]
    default_value: str
    override_prefix: str = "PROVIDER_OVERRIDE_"
    def __init__(self, default_value: str):
        self._provider_config = {}
        self.default_value = default_value
    def load_from_environment(self, env: Mapping[str, str] = None):
        if env is None:
            env = os.environ
        for key, value in env.items():
            if key.startswith(self.override_prefix) and value:
                self.set_provider(key[len(self.override_prefix) :].lower().replace("_", "-"), value)
    def get_provider(self, service: str) -> str: