Builds the exact unsigned proof event for a template, signer pubkey, and created_at:
validates the signer pubkey, created_at, and template (in that order, so the reason is
deterministic when several inputs are bad) and returns a fresh event with cloned tags.
Builds the exact unsigned proof event for a template, signer pubkey, and
created_at: validates the signer pubkey,created_at, and template (in that order, so the reason is deterministic when several inputs are bad) and returns a fresh event with cloned tags.