* SDK: Add `NullSigner` implementation
* SDK: Split `Transaction::verify()` to gain access to results
* CLI: Minor refactor of --sign_only result parsing
* CLI: Enable paritial signing
Signers specified by pubkey, but without a matching --signer arg
supplied fall back to a `NullSigner` when --sign-only is in effect.
This allows their pubkey to be used for TX construction as usual,
but leaves their `sign_message()` a NOP. As such, with --sign-only
in effect, signing and verification must be done separately, with
the latter's per-signature results considered
* CLI: Surface/report missing/bad signers to user
* CLI: Suppress --sign-only JSON output
* nits
* Docs for multi-session offline signing
