geyser: add entries_count field to block metadata

[fanatid]

Problem

v1.16 introduced GeyserPlugin::notify_entry. In Triton One we would like to use it for online data generation in Old Faithful. In our gRPC plugin I added collecting entries for block reconstruction (block meta with all transactions, accounts updates, entries) but I still get errors about the failed reconstruction process. https://github.com/rpcpool/yellowstone-grpc/blob/v1.7.1%2Bsolana.1.16.1/yellowstone-grpc-geyser/src/grpc.rs#L886-L899

Proposed Solution

Add entries_count field to block metadata message — this would be the easiest way to understand how many entries we need to wait before trying to reconstruct the block.