Prototype an approach for generic distance metric syntax in indexes

[Ngalstyan4]

The plan is to never merge this. This is just a hack. But we should discuss the approach and see what we can build on top

[codecov[bot]]

Codecov Report

Merging #38 (e6480ee) into main (8757adb) will increase coverage by 0.46%. The diff coverage is 93.93%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #38 +/- ## ========================================== + Coverage 81.16% 81.63% +0.46% ========================================== Files 10 10 Lines 807 833 +26 Branches 165 172 +7 ========================================== + Hits 655 680 +25 Misses 71 71 - Partials 81 82 +1 ``` | [Files Changed](https://app.codecov.io/gh/lanterndata/lanterndb/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata) | Coverage Δ | | |---|---|---| | [src/hnsw/scan.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvc2Nhbi5j) | `76.19% <0.00%> (ø)` | | | [src/hnsw.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cuYw==) | `82.52% <95.65%> (+2.52%)` | :arrow_up: | | [src/hnsw/distfunc.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvZGlzdGZ1bmMuYw==) | `100.00% <100.00%> (ø)` | |