Add X-Chain to X-Chain transfer support

[felipemadero]

Why this should be merged

This PR adds support for X-Chain to X-Chain transfers in the key transfer command, improving the transfer functionality matrix.

How this works

The implementation follows the same pattern as P-to-P transfers

1. Validation logic updated: Changed from blocking all X-Chain senders to only blocking X-Chain senders when the receiver is NOT X-Chain
2. Destination address handling extended: Added support for X-Chain address format when prompting or retrieving destination addresses from stored keys
3. Transfer flow added: Added conditional to call xToXSend when both sender and receiver are on X-Chain
4. xToXSend function implemented

How this was tested

• Added new E2E test