feat: support signed messages from nano accounts

[mistakia]

Tasks

• [x] feat: add api endpoint for signed message (https://github.com/mistakia/nano-community/pull/110)
• [ ] support signed messages from nano accounts updating data pertaining to their account
• [ ] support signed messages adding content to your own account blocks
• [x] support signed messages directly from account key
• [x] support linking a signing key
  • [x] support signed messages from a linked signing key
  • [x] support invalidating a signing key (https://github.com/mistakia/nano-community/commit/90dc82322d02c9aae78a55b0430fe131b72fcd0e)

account data

• alias
• representative metadata
  • cpu
  • ram
  • disk
  • donation address
  • shutdown notice
  • contact information