Exposed wallet-interface types should be interfaces types and not concrete types

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Developers and implementors want to promote better abstractions and to motivate anything which consume wallet-interface to rely on their local implementations instead of on internal types. The exported types returned by interface functions mentioned in the wallet-interface package should be represented as interfaces as well. For example:

type TransactionRecord struct {
	Txid      string
	Index     uint32
	Value     int64
	Address   string
	Spent     bool
	Timestamp time.Time
}

should become

type TransactionRecord interface {
	Txid()      string
	Index()     uint32
	Value()     int64
	Address()   string
	Spent()     bool
	Timestamp() time.Time
}

Exported Types to migrate:

• [ ] Txn
• [ ] Utxo
• [ ] Stxo
• [ ] Txns
• [ ] Keys
• [ ] FeeLevel
• [ ] TransactionInput
• [ ] TransactionOutput
• [ ] Signature
• [ ] TransactionCallback
• [ ] TransactionRecord