Added a getter for totalShares

[jalextowle]

Description

This PR adds a getter for totalShares that serves as a convenient way for the fuzzing tools (and other integrations) to get the amount of vault shares owned by Hyperdrive for each yield source.

[coveralls]

Pull Request Test Coverage Report for Build 10101943659

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 91.596%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
contracts/src/external/HyperdriveTarget0.sol	0	1	0.0%

Totals
Change from base Build 10082126926:	-0.04%
Covered Lines:	2147
Relevant Lines:	2344

---

💛 - Coveralls

[github-actions]

Hyperdrive Gas Benchmark

Benchmark suite	Current: 81a27ec	Previous: 4ca3f48	Deviation	Status
addLiquidity: min	33893 gas	33893 gas	0%	🟰
addLiquidity: avg	197208 gas	197443 gas	-0.1190%	✅
addLiquidity: max	474728 gas	474728 gas	0%	🟰
checkpoint: min	40316 gas	40316 gas	0%	🟰
checkpoint: avg	144486 gas	144499 gas	-0.0090%	✅
checkpoint: max	256108 gas	256108 gas	0%	🟰
closeLong: min	31384 gas	31384 gas	0%	🟰
closeLong: avg	136087 gas	135945 gas	0.1045%	🚨
closeLong: max	2539399 gas	2539399 gas	0%	🟰
closeShort: min	31327 gas	31327 gas	0%	🟰
closeShort: avg	131198 gas	131311 gas	-0.0861%	✅
closeShort: max	271272 gas	271250 gas	0.0081%	🚨
initialize: min	31305 gas	31305 gas	0%	🟰
initialize: avg	352490 gas	352308 gas	0.0517%	🚨
initialize: max	418737 gas	418715 gas	0.0053%	🚨
openLong: min	33370 gas	33370 gas	0%	🟰
openLong: avg	174168 gas	174213 gas	-0.0258%	✅
openLong: max	333737 gas	333715 gas	0.0066%	🚨
openShort: min	33936 gas	33936 gas	0%	🟰
openShort: avg	174358 gas	174136 gas	0.1275%	🚨
openShort: max	415112 gas	412616 gas	0.6049%	🚨
redeemWithdrawalShares: min	31211 gas	31211 gas	0%	🟰
redeemWithdrawalShares: avg	74852 gas	74990 gas	-0.1840%	✅
redeemWithdrawalShares: max	305184 gas	305162 gas	0.0072%	🚨
removeLiquidity: min	31217 gas	31217 gas	0%	🟰
removeLiquidity: avg	214976 gas	214590 gas	0.1799%	🚨
removeLiquidity: max	403427 gas	403554 gas	-0.0315%	✅

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

[jrhea]

lgtm. just one nit about a test