Add fromJSON and toJSON for transaction payloads

[lassemoldrup]

Purpose

Addresses #307.

Changes

Extends the AccountTransactionHandler interface with fromJSON and toJSON methods for converting transactions from and to JSON-serializable types.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[soerenbf]

Looks good 😄