add support for serverless deployments and session state persis…

[ibakaidov]

• add a pluggable ServerSessionStateStore interface plus an in-memory implementation for persistence
• update streamable handler/transport to load, save, and clear session state so serverless deployments can resume conversations
• document the serverless workflow and add coverage proving state survives between independent handler instances

Testing

• go test ./...

[findleyr]

Hi, I don't think we're likely to accept anything implementing session state persistence until ongoing discussions like SEP-1442 are resolved.