Add overrides for locations

[goncalo-rodrigues]

Multy has a mapping for regions https://github.com/multycloud/multy/blob/4f57fcf3826b13653b9920d6af76bbca2cd773ba/resources/common/cloud_provider.go#L16. In some cases, users might want a different region that is not covered by us.

To do to complete this issue:

• Add override field to https://github.com/multycloud/multy/blob/main/api/proto/commonpb/common.proto#L79
• Use override if specified in https://github.com/multycloud/multy/blob/main/resources/resource_with_id.go#L39 (and check for any other places where it might be accessed directly)
• Add tests - probably something simple like similar to this https://github.com/multycloud/multy/tree/main/test/_configs/virtual_network/virtual_network or also some unit tests