CLI: Add multi-session signing support (#8927) (#8953)

* 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

(cherry picked from commit 98228c392e)

Co-authored-by: Trent Nelson <trent@solana.com>

cli/tests/pay.rs

@@ -1,6 +1,5 @@
 use chrono::prelude::*;
 use serde_json::Value;
-use solana_clap_utils::keypair::presigner_from_pubkey_sigs;
 use solana_cli::{
     cli::{process_command, request_and_confirm_airdrop, CliCommand, CliConfig, PayCommand},
     nonce,
@@ -335,9 +334,11 @@ fn test_offline_pay_tx() {
     check_balance(50, &rpc_client, &config_online.signers[0].pubkey());
     check_balance(0, &rpc_client, &bob_pubkey);
 
-    let (blockhash, signers) = parse_sign_only_reply_string(&sig_response);
-    let offline_presigner =
-        presigner_from_pubkey_sigs(&config_offline.signers[0].pubkey(), &signers).unwrap();
+    let sign_only = parse_sign_only_reply_string(&sig_response);
+    assert!(sign_only.has_all_signers());
+    let offline_presigner = sign_only
+        .presigner_of(&config_offline.signers[0].pubkey())
+        .unwrap();
     let online_pubkey = config_online.signers[0].pubkey();
     config_online.signers = vec![&offline_presigner];
     config_online.command = CliCommand::Pay(PayCommand {