extrinsics: fix Vec<AccountId> args

[ascjones]

Where a message or a constructor accepts a Vec<AccountId> as an argument e.g multisig, the custom decoder should be used so that SS58 format. e.g.

 contract instantiate \
      --args 2 [5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY,5FHneW46xGXgs5mUiveU4sbTyGBzmstUspZC92UhjJM694ty] \
      --suri //Alice \
      --dry-run

[cmichi]

@ascjones What about (AccountId, …)?

[ascjones]

@ascjones What about (AccountId, …)?

You mean composite types containing an AccountId? Already handled: https://github.com/paritytech/cargo-contract/blob/cfd55bd66811f959e465bd7f1b6d5d60e2240fbc/src/cmd/extrinsics/transcode/transcoder.rs#L583