Implement standalone BuyDirect msg

[technicallyty]

Summary

Currently, the Buy msg has two options to submit buy orders, one of which acts more like BuyDirect and the other for DEX purposes.

Issues

There are a few UX quirks as a result of this:

• we have two fields disable_partial_fill and expiration that do not apply to the message. These fields would probably be ignored, even if supplied.
• the two types of orders are selected via a OneOf and will need to be refactored anyways, as this causes issues for amino signing.

Proposal

Create a standalone BuyDirect RPC endpoint, separate from the current monolithic Buy

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[ryanchristo]

I support this. Less complexity and cleaner solution to the amino signing issue. Also prevents breaking changes when we introduce the orderbook model. Essentially Buy would be create buy order and Sell would be create sell order, BuyDirect would not create a buy order and not include disable_partial_fills and expiration.

[ryanchristo]

We should probably get a second review for #899 and merge, then implement this as a followup.

[aaronc]

I support this. Less complexity and cleaner solution to the amino signing issue. Also prevents breaking changes when we introduce the orderbook model. Essentially Buy would be create buy order and Sell would be create sell order, BuyDirect would not create a buy order and not include disable_partial_fills and expiration.

When there is an order book approach, BuyDirect will create a buy order, but it should either succeed or fail in the next batch so expiration isn't needed. I think separate methods are okay