Removing Storage::backing method

[citizen-stig]

Description

As it heavily leaks abstraction.

Replace it with fn get_with_proof(&self, key: StorageKey) -> Option<StorageProof<Self::Proof>> which at least does not expose underlying storage.

Testing

Current tests are passing

Docs

No updates in documentation

[codecov]

Codecov Report

Merging #978 (121e3fa) into nightly (ff21c4a) will decrease coverage by 0.2%.
Report is 3 commits behind head on nightly.
The diff coverage is 45.4%.

Files	Coverage Δ
...mplementations/sov-attester-incentives/src/call.rs	87.0% <100.0%> (-0.1%)	⬇️
module-system/sov-state/src/storage.rs	94.3% <ø> (ø)
module-system/sov-state/src/zk_storage.rs	59.1% <ø> (+0.8%)	⬆️
...plementations/sov-attester-incentives/src/query.rs	58.1% <83.3%> (-22.9%)	⬇️
module-system/sov-state/src/prover_storage.rs	95.1% <80.0%> (-0.2%)	⬇️
...ule-system/sov-modules-api/src/state/scratchpad.rs	88.1% <0.0%> (-3.2%)	⬇️

... and 19 files with indirect coverage changes

[preston-evans98]

We do need the get_bond_proof method for optimistic rollups (and in other places), so I would definitely prefer not to remove it. If we don't remove it, what options do we have to unblock you?