feat: filter registered subnets internal only

[sebastiendan]

Description

This PR adds a filter in the useRegisteredSubnets hook so that only Incal gets returned, as a temporary solution for external subnets not to be used in the dapp (temporary solution until we have permissionless subnet registration).

Fixes TOO-359

PR Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated tests that comprehensively prove my change is effective or that my feature works