feat: add bytes_val to TemplateValue for raw binary expression params

proto3 forbids raw binary in a string field (UTF-8 enforced), so a client
pre-built membership-filter blob (bloom/roaring/bitset) currently has to be
base64-encoded onto string_val (~33% inflation). Add a bytes bytes_val = 6
variant to the TemplateValue oneof so such blobs travel as raw bytes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: xiaofanluan <xf@hjjaq.com>

Author: xiaofanluan