architecture/rpc_api.md: actualize proc_sql

Summary

• architecture/rpc_api.md: actualize proc_sql
• architecture/rpc_api.md: add proc_sql_execute

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• Close #288 (closed)
• Cherry-pick to: 24.4
• Staging: https://docs.binary.picodata.io/picodata/eivkov_proc_sql_dispatch/
• Follow-up for picodata!1072 (merged) and picodata!1083 (merged)