[ENH][SEC]: Move HNSW Index metadata (pickle) to protobuf binary file

[tazarov]

Description of changes

Summarize the changes made by this PR.

• Improvements & Bug fixes
  • We persist the HNSW max_seq_id when the index is written; this will help with WAL cleanup and will not require loading the HNSW index before cleanup.

Test plan

How are these changes tested?

• [x] Tests pass locally with pytest for python

Documentation Changes

N/A