SignedRequest typedef
A whole Trust Task request document — id, type, issuer, payload and every
other member, proof included — exactly as the ballot box received it, never
re-serialised: its proof is taken over its members, and a re-serialised copy does
not reliably reproduce them. Typed as an open object so every member survives;
CONVENTIONS §9.6 gives the checks that make it a trustee's step.
Implementation
typedef SignedRequest = Map<String, dynamic>;