M1: Configure and use multiple xpubs on the same Substrate account

[mgravitt]

The user should be able to add multiple xpubs to their Substrate account, each under a specific label.

For example, Alice may want to participate in vaults with different devices with labels such as coldcard, hancock, my-own-cryptic-label, or have different xpubs used in different groups, such as personal and business.

The changes needed to support this scenario would be:

• Additional pallet storage for the extra relationship and label
• UI components for adding, editing, and deleting labeled xpubs
• UI updates for querying and displaying lists of labeled xpubs when a user is creating a vault
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