Implement `sdk.Msg` interface for `MsgRegisterAccount`

[damiannolan]

Summary

Implement the sdk.Msg interface for the new MsgRegisterAccount type with associated test cases.

• ValidateBasic() error
• GetSigners() []sdk.AccAddress

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[gjermundgaraba]

This issue was mistakenly reopened during some integration work, so closing again now.