Title: stateful session enhancement: to support the MCP protocol session management

Description:

See https://github.com/modelcontextprotocol/specification/blob/main/docs/specification/2025-03-26/basic/transports.md#session-management

MCP protocol is become more and more popular for AI agent developing. The session affinity is very important part for proxying MCP traffic becase it's stateful protocol.

But fortunately, Envoy have provided the stateful sesssion filter which could provide a stateful session support based on the header or cookie. This would be great if we can support the MCP session affinity support.

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Any extra documentation required to understand the issue.