Research: Store TX memo

[maikReal]

We need to have the ability to store tx memo somewhere.
In parallel we also work on UI here:

• Tx memo zkbob-ui#59

TODO:

• Think about formats of a memo

[maikReal]

For UX: Just add a text field to the transaction tab + show the memo in a history screen

[EvgenKor]

Think about formats of a memo

First proposals for low-level implementation: https://hackmd.io/@Evgen/B1CYzbf7i

[maikReal]

@EvgenKor the requirements for the first iteration:

1. Information that will be stored in tx memo – only text without any other attachments
2. TX memo will be available only for single transfer in the first iteration (if it's easy to do for multitransfer as well, then we can support tx memo for both types)
3. Better to encrypt data. But only if it does not seriously affect the transfer speed