Enhance Security: Restrict GnoKey to Official Chains

[iuricmp]

During the Contributors Call, Jae raised a concern regarding the security of Gnokey. Specifically, there is currently no mechanism to prevent users from interacting with non-official chains. To enhance security, we should consider implementing a solution that restricts Gnokey to only interact with an official list of chains

Proposed Solution

• Maintain an official list of chains in a GitHub repository.
• Gnokey should fetch and validate against this list before allowing interactions.

Additional Considerations

• Should there be an override flag for developers/testnets?

[jefft0]

This could be achieved by simply removing the "Custom chain" button. But this button is pretty useful (and Adena also has it).