extrinsics: fix `Vec<AccountId>` args

Sergej Kostjucenko requested to merge aj/vec-args into master

Created by: 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

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