Add `get_spot_price_after_short` method to rust sdk

[DannyDelott]

The ability to know what the spot price is after opening a short is helpful, and there is prior art for this in open long.

Prior art:
https://github.com/delvtech/hyperdrive/blob/main/crates/hyperdrive-math/src/long/open.rs#L39

Having this capability for shorts would allow arbers to know how big of a short they need to open in order to bring the fixed rate back up to the vault rate.

[akhamisa]

Hey @DannyDelott originally the intention was to have the user input the target market fixed rate and have the UI return the position size required to move the fixed rate in line with the target (presumably the same as the var rate). This functionality would Ideally be available in a new section in the Short open position modal (below the Preview tx section).

Charles/Jonny confirmed we're cool to do it they way you described for now which is:

1. User enters position size in the input field in the Short open position modal (no change there)
2. In the preview tx section below, a new line is introduced with the "Market rate impact" [resulting market rate if the position were to be opened]

We can consider the original implementation for a later time post testnet if users feel strongly about it.