Monero/XMR: Allow tx keys to be imported, manually and/or from other wallet formats

[jivanpal]

Cake Wallet currently supports exporting a wallet and its transaction data, including user-specified notes, to file, and supports importing such an file. However, if a user is migrating to Cake Wallet from another wallet, such as Monerujo, Feather, or Monero GUI Wallet, then they will not be able to carry over the tx keys for transactions where they have spent funds, and thus will not be able to prove such spending to another person in future if they no longer use the other wallet app.

As a minimum measure, users should be able to manually enter tx keys associated with a transaction. For additional convenience, importing a simple, standard file format with these data (e.g. a simple XML or JSON document) for several transactions should be supported, and perhaps even supporting importing this data from the export formats of other wallets, such as those listed above.