Adding zeroize on drop for VRFExpandedPrivateKey

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03% :warning:

Comparison is base (5c2bc73) 87.77% compared to head (b0d467a) 87.74%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #403 +/- ## ========================================== - Coverage 87.77% 87.74% -0.03% ========================================== Files 39 39 Lines 8771 8775 +4 ========================================== + Hits 7699 7700 +1 - Misses 1072 1075 +3 ``` | [Files Changed](https://app.codecov.io/gh/facebook/akd/pull/403?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=facebook) | Coverage Δ | | |---|---|---| | [akd\_core/src/ecvrf/ecvrf\_impl.rs](https://app.codecov.io/gh/facebook/akd/pull/403?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=facebook#diff-YWtkX2NvcmUvc3JjL2VjdnJmL2VjdnJmX2ltcGwucnM=) | `92.07% <100.00%> (+0.14%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/facebook/akd/pull/403/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=facebook)

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