refactor!: change GetSigners return type to []sdk.AccAddress (#9915)

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## Description
Changes the `sdk.Msg` interface method `GetSigners() []string` to `GetSigners() []sdk.AccAddress`

Closes: #9885 


+change GetSigner return type
+defer address checking to each msg's `ValidateBasic`  method
+clean up consistency/redundancy issues in validate basic


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server/mock/tx.go

@@ -58,7 +58,7 @@ func (tx kvstoreTx) ValidateBasic() error {
 	return nil
 }
 
-func (tx kvstoreTx) GetSigners() []string {
+func (tx kvstoreTx) GetSigners() []sdk.AccAddress {
 	return nil
 }
 

server/rosetta/converter.go

@@ -254,7 +254,7 @@ func (c converter) Ops(status string, msg sdk.Msg) ([]*rosettatypes.Operation, e
 		op := &rosettatypes.Operation{
 			Type:     opName,
 			Status:   &status,
-			Account:  &rosettatypes.AccountIdentifier{Address: signer},
+			Account:  &rosettatypes.AccountIdentifier{Address: signer.String()},
 			Metadata: meta,
 		}