Add a command to download a tx into tx.json file that can be used by `ckb-cli mock-tx dump`

[doitian]

The command ckb-cli mock-tx dump requires an input tx file, but there's no corresponding sub command to download a tx.json from the CKB node.

Another option is let ckb-cli mock-tx dump accepts an alternative option to dump a tx from CKB node via tx hash.

Once this is available, it can be used as a transaction dumper for ckb-debugger.

[doitian]

May related: https://github.com/nervosnetwork/ckb-cli/issues/577

[github-actions[bot]]

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