Add memo to transaction creation

[MartinMinkov]

Description
Adds the memo property to various methods in SnarkyJS. The following methods now have a memo property:

1. signFeePayer
2. addFeePayer
3. Mina.transaction
4. Deploy

These changes allow attaching a memo to a Parties structure. If there is no memo passed into these methods, they will simply reuse the memo property in the Parties structure.

All the code to create a signature with the memo field is already included in the backend OCaml code. It just needed to be exposed in SnarkyJS.

Tested By
Manual testing

[mitschabaude]

@MartinMinkov This is a good addition, but doesn't yet expose the memo field to the most important user API for creating transactions, which is Mina.transaction. It would be cool to also have an optional memo field in the (optional) first argument to Mina.transaction

[mitschabaude]

LGTM!