feat: add useMultipart flag

[friendlymatthew]

Closes #158

In our init(), this PR introduces a Config which contains the following:

export type Config = {
     useMultipartByteRanges?: boolean
}

We drill this through and in range-request.ts, we add logic where if that flag is either undefined or false, we'll resolve each range individually, else we'll continue with multi-fetching. If we reach a 200 response when multi-fetching, we'll warn the user that this flag hasn't been set.

Tested via local

[github-actions]

Benchmark results: https://github.com/kevmo314/appendable/actions/runs/8144146232/artifacts/1295682913