[btree] Add support for prefix range scan on btree global index

[ArnavBalyan]

Purpose

• Prefix lookup currently has no direct path and returns every non null row, every prefix query reads the whole segment.
• Add query_prefix on the btree reader, which resolves a prefix on range [prefix, prefix_successor(prefix)).

Tests

• UT added that covers multi match, disjoint and empty prefix.

[JingsongLi]

+1