int8 support in diskann-garnet/vectorset

[harsha-simhadri]

Add int8 data type to diskann-garnet/vectorset as it is the second most popular data type.

Currently we see

S Q:\DiskANN\vectorset> cargo run --release -- --config .\config.toml.example --data-type int8 ingest --tasks 32 --degree 48 --l-build 300 data.bin                                                                     
error: invalid value 'int8' for '--data-type <DATA_TYPE>'
  [possible values: uint8, float32]