Add get_spot_price_after_short method to rust-sdk

[sentilesdal]

There was already a similar method get_spot_price_after_long so this follows the pattern set there. This fixes #753.

[github-actions]

Hyperdrive Gas Benchmark

Benchmark suite	Current: 0d12bcd	Previous: 53bce0d	Deviation	Status
addLiquidity: min	1546 gas	1546 gas	0%	🟰
addLiquidity: avg	67508 gas	68099 gas	-0.8679%	✅
addLiquidity: max	292728 gas	292728 gas	0%	🟰
checkpoint: min	1150 gas	1150 gas	0%	🟰
checkpoint: avg	48406 gas	48288 gas	0.2444%	🚨
checkpoint: max	191173 gas	191173 gas	0%	🟰
closeLong: min	1492 gas	1492 gas	0%	🟰
closeLong: avg	28428 gas	28378 gas	0.1762%	🚨
closeLong: max	152268 gas	151400 gas	0.5733%	🚨
closeShort: min	1494 gas	1494 gas	0%	🟰
closeShort: avg	30225 gas	30237 gas	-0.0397%	✅
closeShort: max	146093 gas	129747 gas	12.5984%	🚨
initialize: min	1495 gas	1495 gas	0%	🟰
initialize: avg	213307 gas	213307 gas	0%	🟰
initialize: max	253585 gas	253585 gas	0%	🟰
openLong: min	1432 gas	1432 gas	0%	🟰
openLong: avg	50744 gas	50733 gas	0.0217%	🚨
openLong: max	187405 gas	187405 gas	0%	🟰
openShort: min	1520 gas	1520 gas	0%	🟰
openShort: avg	49987 gas	49993 gas	-0.0120%	✅
openShort: max	180144 gas	180144 gas	0%	🟰
redeemWithdrawalShares: min	1488 gas	1488 gas	0%	🟰
redeemWithdrawalShares: avg	20389 gas	19937 gas	2.2671%	🚨
redeemWithdrawalShares: max	108516 gas	108516 gas	0%	🟰
removeLiquidity: min	1552 gas	1552 gas	0%	🟰
removeLiquidity: avg	149141 gas	150291 gas	-0.7652%	✅
removeLiquidity: max	324881 gas	324881 gas	0%	🟰

This comment was automatically generated by workflow using github-action-benchmark.

[jalextowle]

LGTM