Add support for vector search in SAI

The patch associated with this ticket adds a new vector index to SAI. This introduces the following new elements and changes to SAI:

• VectorMemtableIndex - the in-memory representation of the vector indexes that writes data to a DiskANN instance
• VectorSegmentBuilder - that writes a DiskANN graph to the following on-disk components:
  • VECTOR - contains the floating point vectors associated with the graph
  • TERMS - contains the HNSW graph on-disk representation written by a HnswGraphWriter
  • POSTINGS - contains the index postings as written by a VectorPostingsWriter
• VectorIndexSegmentSearcher - used to search the on-disk DiskANN graph