refactor: CustomGetSigners for MsgSwapOrder

[poorphd]

Description

Cosmos SDK v0.50 this method has been dropped and replaced with the declarative cosmos.msg.v1.signer protobuf tag.
For MsgSwapOrder, which does not have a cosmos.msg.v1.signer to
resolve the signer, custom GetSigners function is defined.

Closes: code-423n4/2024-05-canto-findings#27

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[dudong2]

LGTM