Decouple client/tx from x/auth

[aaronc]

As part of #5864 part 2), this PR decouples the new client/tx package from x/auth (as an incremental step to deprecating all the tx logic in x/auth) and adds StdFee, StdSignature, StdSignMsgBase and StdTxBase to codec/std, while deprecating the corresponding types in x/auth

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[aaronc]

This is R4R. The failure on circleci proto is expected because this introduces breaking proto changes.

[alexanderbez]

LGTM 👍