Add optional size-limited memo field to transactions

[cwgoes]

We should add an optional size-limited memo field to transactions (say, limited to 1 KB), which can be filled with arbitrary data (which may be encrypted, likely in the case of a shielded transfer). This can be used to communicate e.g. an identifier corresponding to a payment to the recipient.

/ref https://forum.namada.net/t/messages-in-transactions/375

[Fraccaman]

to any transactions?

[cwgoes]

to any transactions?

Yes.