wallet-rpc: watch-only and cold wallet features added

- unsigned_txset, signed_txset in transfer / submit_transfer / sign_transfer
- export_outputs, import_outputs

Squashed commits:
[f4d9f3d4] wallet-rpc: do_not_relay removed from submit_transfer
[5b16a86f] wallet-rpc: review-fix - method signature changes, renaming
[b7fbb10a] wallet-rpc: naming fixes (unsigned vs signed), consts renamed
[8c7d2727] wallet-rpc: sign_transfer added
[481d024a] wallet2: sign_tx splitted to work with strings and structs, more granular
[2a474db9] wallet-rpc: wallet2::load_unsigned_tx split to load from str, file
[b1e3a018] wallet-rpc: review fix, load_tx_from_str variable rename
[1f6373be] wallet-rpc: review fix: save_tx_to_{str,file}
[2a08eafc] wallet-rpc: review comments fixes
- redundant this removed from wallet2.cpp
- load_tx_from_str, load_tx_from_file
[43498052] wallet-rpc: submit_transfer added
[9c45d1ad] wallet-rpc: watch_only check, return unsigned_txset
[62831396] wallet2: added string variants to load_tx, save_tx

- analogously to save_multisig_tx
- required for monero-wallet-rpc to support watch-only wallet

src/wallet/wallet_rpc_server_error_codes.h

@@ -69,3 +69,7 @@
 #define WALLET_RPC_ERROR_CODE_MULTISIG_SUBMISSION    -36
 #define WALLET_RPC_ERROR_CODE_NOT_ENOUGH_UNLOCKED_MONEY -37
 #define WALLET_RPC_ERROR_CODE_NO_DAEMON_CONNECTION   -38
+#define WALLET_RPC_ERROR_CODE_BAD_UNSIGNED_TX_DATA   -39
+#define WALLET_RPC_ERROR_CODE_BAD_SIGNED_TX_DATA     -40
+#define WALLET_RPC_ERROR_CODE_SIGNED_SUBMISSION      -41
+#define WALLET_RPC_ERROR_CODE_SIGN_UNSIGNED          -42