feat: add protocol fields for conversation sync optimization

[redflyingfish]

🅰 Please add the issue ID after "Fixes #"

Fixes #3496

Summary

Add protocol support for optimizing full owner conversation sync.

Changes:

• add optional pagination to GetFullOwnerConversationIDsReq;
• add total to GetFullOwnerConversationIDsResp;
• add a dedicated GetConversationsFullSyncSeqs RPC;
• add dedicated full-sync sequence messages so userMinSeq is only exposed on the full-sync path.

Motivation

This prepares the protocol layer for reducing unnecessary historical conversation synchronization when a user logs in from a fresh device.

The new RPC separates full-sync sequence data from the existing GetConversationsHasReadAndMaxSeq response, so ordinary seq queries do not need to include userMinSeq.

Compatibility

This change is backward-compatible:

• existing request and response fields are unchanged;
• new fields are optional/additive;
• existing RPCs keep their current request and response types.

Related

Part of openimsdk/open-im-server#3496.

Tests

go test ./...