expose set/get walletcache attribute functionality in wallet api

src/wallet/api/wallet.cpp

@@ -1697,6 +1697,17 @@ void WalletImpl::setDefaultMixin(uint32_t arg)
     m_wallet->default_mixin(arg);
 }
 
+bool WalletImpl::setCacheAttribute(const std::string &key, const std::string &val)
+{
+    m_wallet->set_attribute(key, val);
+    return true;
+}
+
+std::string WalletImpl::getCacheAttribute(const std::string &key) const
+{
+    return m_wallet->get_attribute(key);
+}
+
 bool WalletImpl::setUserNote(const std::string &txid, const std::string &note)
 {
     cryptonote::blobdata txid_data;

src/wallet/api/wallet.h

@@ -168,6 +168,10 @@ public:
     virtual void setListener(WalletListener * l) override;
     virtual uint32_t defaultMixin() const override;
     virtual void setDefaultMixin(uint32_t arg) override;
+
+    virtual bool setCacheAttribute(const std::string &key, const std::string &val) override;
+    virtual std::string getCacheAttribute(const std::string &key) const override;
+
     virtual bool setUserNote(const std::string &txid, const std::string &note) override;
     virtual std::string getUserNote(const std::string &txid) const override;
     virtual std::string getTxKey(const std::string &txid) const override;

src/wallet/api/wallet2_api.h

@@ -890,6 +890,19 @@ struct Wallet
      */
     virtual void setDefaultMixin(uint32_t arg) = 0;
 
+    /*!
+     * \brief setCacheAttribute - attach an arbitrary string to a wallet cache attribute
+     * \param key - the key
+     * \param val - the value
+     * \return true if successful, false otherwise
+     */
+    virtual bool setCacheAttribute(const std::string &key, const std::string &val) = 0;
+    /*!
+     * \brief getCacheAttribute - return an arbitrary string attached to a wallet cache attribute
+     * \param key - the key
+     * \return the attached string, or empty string if there is none
+     */
+    virtual std::string getCacheAttribute(const std::string &key) const = 0;
     /*!
      * \brief setUserNote - attach an arbitrary string note to a txid
      * \param txid - the transaction id to attach the note to