wallet: Replace use of purpose strings with an enum

Instead of storing and passing around fixed strings for the purpose of
an address, use an enum.

This also rationalizes the CAddressBookData struct, documenting all fields and
making them public, and simplifying the representation to avoid bugs like
https://github.com/bitcoin/bitcoin/pull/26761#discussion_r1134615114 and make
it not possible to invalid address data like change addresses with labels.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/qt/addresstablemodel.h

@@ -5,6 +5,8 @@
 #ifndef BITCOIN_QT_ADDRESSTABLEMODEL_H
 #define BITCOIN_QT_ADDRESSTABLEMODEL_H
 
+#include <optional>
+
 #include <QAbstractTableModel>
 #include <QStringList>
 
@@ -16,6 +18,9 @@ class WalletModel;
 namespace interfaces {
 class Wallet;
 }
+namespace wallet {
+enum class AddressPurpose;
+} // namespace wallet
 
 /**
    Qt model of the address book in the core. This allows views to access and modify the address book.
@@ -71,7 +76,7 @@ public:
     QString labelForAddress(const QString &address) const;
 
     /** Look up purpose for address in address book, if not found return empty string. */
-    QString purposeForAddress(const QString &address) const;
+    std::optional<wallet::AddressPurpose> purposeForAddress(const QString &address) const;
 
     /* Look up row index of an address in the model.
        Return -1 if not found.
@@ -89,7 +94,7 @@ private:
     EditStatus editStatus = OK;
 
     /** Look up address book data given an address string. */
-    bool getAddressData(const QString &address, std::string* name, std::string* purpose) const;
+    bool getAddressData(const QString &address, std::string* name, wallet::AddressPurpose* purpose) const;
 
     /** Notify listeners that data changed. */
     void emitDataChanged(int index);
@@ -97,7 +102,7 @@ private:
 public Q_SLOTS:
     /* Update address list from core.
      */
-    void updateEntry(const QString &address, const QString &label, bool isMine, const QString &purpose, int status);
+    void updateEntry(const QString &address, const QString &label, bool isMine, wallet::AddressPurpose purpose, int status);
 
     friend class AddressTablePriv;
 };