added support for dumping serialized transaction

[kagren]

The current CLI does not support dumping a serialized transaction, which is a problem for stake pool operators that use multisig wallets. This PR adds a new CLI option, --dump-transaction-message-only, that will instead of sending the transaction just print the transaction to stdout.

By setting that option, it also becomes possible to pass a pubkey to the CLI arguments that normally expect a keypair for signing.

[joncinque]

Thanks for your contribution!

The scaffolding already exists for all this in a simpler way, using the offline_args module. I think your PR took some inspiration from it, but we can add the offline_args_config() parameter, as with: https://github.com/solana-program/token-2022/blob/075f4178d752ddd4a0564f9b1cae079cbed38998/clients/cli/src/clap_app.rs#L1105

And then we just need to add sign_only / dump_transaction_message as config-level fields, as in https://github.com/solana-program/token-2022/blob/075f4178d752ddd4a0564f9b1cae079cbed38998/clients/cli/src/config.rs#L85

And then we can remove DummySigner and use NullSigner from the SDK instead, as in https://github.com/solana-program/token-2022/blob/075f4178d752ddd4a0564f9b1cae079cbed38998/clients/cli/src/config.rs#L603

And finally, rather than encoding in base58 by hand, we can use return_signers_data: https://github.com/anza-xyz/agave/blob/d2ba8b8160c209d13d1fcb630f0e856e0bd14114/cli-output/src/cli_output.rs#L2663

NOTE: return_signers_data encodes the message as base64, and not base58

How does that sound?