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

cli/src/display.rs

@@ -1,6 +1,6 @@
 use crate::cli::SettingType;
 use console::style;
-use solana_sdk::transaction::Transaction;
+use solana_sdk::hash::Hash;
 
 // Pretty print a "name value"
 pub fn println_name_value(name: &str, value: &str) {
@@ -27,13 +27,25 @@ pub fn println_name_value_or(name: &str, value: &str, setting_type: SettingType)
     );
 }
 
-pub fn println_signers(tx: &Transaction) {
+pub fn println_signers(
+    blockhash: &Hash,
+    signers: &[String],
+    absent: &[String],
+    bad_sig: &[String],
+) {
     println!();
-    println!("Blockhash: {}", tx.message.recent_blockhash);
-    println!("Signers (Pubkey=Signature):");
-    tx.signatures
-        .iter()
-        .zip(tx.message.account_keys.clone())
-        .for_each(|(signature, pubkey)| println!("  {:?}={:?}", pubkey, signature));
+    println!("Blockhash: {}", blockhash);
+    if !signers.is_empty() {
+        println!("Signers (Pubkey=Signature):");
+        signers.iter().for_each(|signer| println!("  {}", signer))
+    }
+    if !absent.is_empty() {
+        println!("Absent Signers (Pubkey):");
+        absent.iter().for_each(|pubkey| println!("  {}", pubkey))
+    }
+    if !bad_sig.is_empty() {
+        println!("Bad Signatures (Pubkey):");
+        bad_sig.iter().for_each(|pubkey| println!("  {}", pubkey))
+    }
     println!();
 }