CLI: Add multi-session signing support (#8927)

* 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

sdk/src/transaction.rs

@@ -300,14 +300,20 @@ impl Transaction {
         Ok(())
     }
 
-    /// Verify the transaction
-    pub fn verify(&self) -> Result<()> {
-        if !self
-            .signatures
+    pub fn verify_with_results(&self) -> Vec<bool> {
+        self.signatures
             .iter()
             .zip(&self.message.account_keys)
             .map(|(signature, pubkey)| signature.verify(pubkey.as_ref(), &self.message_data()))
-            .all(|verify_result| verify_result)
+            .collect()
+    }
+
+    /// Verify the transaction
+    pub fn verify(&self) -> Result<()> {
+        if !self
+            .verify_with_results()
+            .iter()
+            .all(|verify_result| *verify_result)
         {
             Err(TransactionError::SignatureFailure)
         } else {