Mailbox impl

[kentbull]

This is a mailbox implementation that:

• supports the existing "multipart/form-data" format of kli mailbox add
• ensures /fwd messages are only supported for recipient AIDs where the mailbox hab has a supporting end role auth for that recipientAID with AuthorizedForwardHandler.
• mailbox add/remove on mailbox host with MailboxAddRemoveEnd HTTP endpoint, compatible with kli mailbox add
  • kli mailbox remove does not exist yet though when it does the MailboxAddRemoveEnd will be ready for it.
• delkel in kli mailbox add: Delegated AIDs can send mailbox add request with kli mailbox add
• Auto-make mailbox non-transferable AID on startup if the one identified by --name and --alias does not exist.
• Adds a test script showing how the mailbox works by facilitating challenge response.

AI Sourcing Note:
Codex assisted creation of this PR. I reviewed everything and adjusted things here and there, though it was drafted by Codex with my prompts.