feat: add sqllite as an option to store signing keys

[1xstj]

Summary of changes Changes introduced in this pull request:

Reference issue to close (if applicable) Closes #684 #716

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (d5512ea) 18.79% compared to head (7bb9c2d) 18.77%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #723 +/- ## ========================================== - Coverage 18.79% 18.77% -0.01% ========================================== Files 80 80 Lines 5958 5963 +5 ========================================== Hits 1119 1119 - Misses 4839 4844 +5 ``` | [Files](https://app.codecov.io/gh/webb-tools/dkg-substrate/pull/723?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webb-tools) | Coverage Δ | | |---|---|---| | [dkg-primitives/src/types.rs](https://app.codecov.io/gh/webb-tools/dkg-substrate/pull/723?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webb-tools#diff-ZGtnLXByaW1pdGl2ZXMvc3JjL3R5cGVzLnJz) | `0.00% <0.00%> (ø)` | | | [dkg-gadget/src/lib.rs](https://app.codecov.io/gh/webb-tools/dkg-substrate/pull/723?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webb-tools#diff-ZGtnLWdhZGdldC9zcmMvbGliLnJz) | `0.00% <0.00%> (ø)` | |

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