wallet-core: Add creation of phoenix-transaction that increases the max spendable

[moCello]

Summary

Add a phoenix transaction that merges unspent notes to increase the max spendable.

Possible solution design or implementation

The transaction should take the 4 highest unspent notes and create a transaction to self so that these notes are merged.

Additional context

In phoenix, we use the notion of spendable (vs total), to express the highest amount that is spendable in one transaction.

This value will be different from the total phoenix DUSK owned by a key if the key owns more than 4 unspent notes. This is due to the fact that in order for the transaction circuits not to blow up in size, we can only create phoenix-transactions with at most 4 input notes.

To mitigate this, we already implement a strategy to maximize the amount of notes spent in each transaction in our pick_notes function.

If the user wants to spend more than the spendable however, notes need to be merged.