Allow service Msg's to gracefully support Amino JSON

[aaronc]

There is a desire to migrate all CLI methods to use service Msg's (#8306), but this will mean that they no longer support amino JSON with the current signing logic.

Gracefully converting ServiceMsgs to Amino JSON is a relatively small change and simply involves casting the ServiceMsg.Request to sdk.Msg in the legacy sign mode handler and the legacy client CopyTx code.

I suggest we do this for v0.41 so that we can migrate to ServiceMsg everywhere but not break the ledger app (because SIGN_MODE_TEXTUAL likely won't be ready).