Fix search states of CCS requests in mixed cluster

[dnhatn]

Forward-port of #70948 to 8.0

Previously, the search states are stored in ReaderContext on data nodes. Since 7.10, we send them to the coordinating node in a QuerySearchResult of a `ShardSearchRequest and the coordinating node then sends them back in ShardFetchSearchRequest. We must keep the search states in data nodes unless they are sent back in the fetch phase. We used the channel version to determine this guarantee. However, it's not correct in CCS requests in mixed clusters.

1. The coordinating node of the local cluster on the old version sends a ShardSearchRequest to a proxy node of the remote cluster on the new version. That proxy node delivers the request to the data node. In this case, the channel version between the data node and the proxy node is >= 7.10, but we won't receive the search states in the fetch phase as they are stripped out in the channel between the old coordinating node and the new proxy.

[coordinating node v7.9] --> [proxy node v7.10] --> [data node on v7.10]

2. The coordinating node of the local on the new version sends a ShardSearchRequest to a proxy node of the remote cluster on the new version. However, the coordinating node sends a ShardFetchSearchRequest to another proxy node of the remote cluster that is still on an old version. The search states then are stripped out and never reach the data node.

- query phase: [coordinating node v7.10] --> [proxy node v7.10] --> [data node on v7.10]
- fetch phase: [coordinating node v7.10] --> [proxy node v7.9] --> [data node on v7.10]

This commit fixes the first issue by adding an explicit flag keepSearchStatesInContext to ShardSearchRequest and the second by continue storing the search states in ReaderContext unless all nodes are upgraded.

Relates #52741

[dnhatn]

@elasticmachine test this please