expose downsampleRaw function for testing purpose

[yeya24]

Now in Thanos codebase, it is hard to create a downsampled block for testing directly. However, this is very much needed in the E2E tests or other unit tests.

I propose to export downsample.downsampleRaw method. The current code is:

func downsampleRaw(data []sample, resolution int64) []chunks.Meta {
	if len(data) == 0 {
		return nil
	}

	mint, maxt := data[0].t, data[len(data)-1].t
	// We assume a raw resolution of 1 minute. In practice it will often be lower
	// but this is sufficient for our heuristic to produce well-sized chunks.
	numChunks := targetChunkCount(mint, maxt, 1*60*1000, resolution, len(data))
	return downsampleRawLoop(data, resolution, numChunks)
}

downsampleRaw will be renamed to DownsampleRaw and data []sample will be changed to data []tsdbutil.Sample. This is an interface type and is much better for usability. Any thoughts about it?