feat: make Metric the basic unit for metrics, not str

[stephantul]

We used a string as the basic unit for all metrics, but we already have a really nice way of expressing which metrics we should use: the Metric Enum.

This PR changes all stored metrics to be Metric Enum values, and not backend-specific strings. This makes Enum values comparable over backends.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
vicinity/backends/basic.py	66.66%	2 Missing ⚠️
vicinity/backends/voyager.py	80.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
vicinity/backends/annoy.py	95.18% <100.00%> (+0.24%)	⬆️
vicinity/backends/base.py	85.50% <100.00%> (+2.45%)	⬆️
vicinity/backends/faiss.py	89.74% <100.00%> (ø)
vicinity/backends/hnsw.py	95.38% <100.00%> (-2.90%)	⬇️
vicinity/backends/pynndescent.py	95.52% <100.00%> (-1.26%)	⬇️
vicinity/backends/usearch.py	96.92% <100.00%> (ø)
vicinity/backends/voyager.py	96.77% <80.00%> (ø)
vicinity/backends/basic.py	77.86% <66.66%> (-12.46%)	⬇️

... and 1 file with indirect coverage changes