RDDT: Avoid materializing string views on group by

[hicder-rddt]

3x reduction in both CPU and p99 latency for feeds-posts-3

One of the inefficiency I just found in Milvus: during group-by, let's say a segment has 1 million docs, and we need to get document 5, 10, 20. Currently, Milvus will:

1. Construct a vector of 1 million elements, each with a "view" (offset, len) into the segment's data. Let's call this vector "string_views".
   a. On a high level, the way is does is: for 0 to 1M, string_views.push_back({offsets[i], len[i]})
2. For each element (5, 10, 20), Milvus will read string_views[5], string_views[10], string_views[20] respectively, and return the corresponding documents.

From our flamegraph, step 1 takes 66% of the CPU! However, it's a waste! All we need to do is: read ({offsets[5], len[5]}, {offsets[10], len[10]}, {offsets[20], len[20]}), then return the corresponding documents.

Result: we managed to reduce CPU usage by 3x, and latency also reduced by 3x!