[Schema][SEP-1319] Decouple Request Payload from RPC Methods Definition

[chr-hertel]

Issue to address and track SEP-1319:

This SEP proposes a structural refactoring of the Model Context Protocol (MCP) specification. The core change is to define payload of requests (e.g., CallToolRequest) as independent definitions and have the RPC method definitions refer to these models. This decouples the definition of the data payload from the definition of the remote procedure that transports it, leading to a clearer, more modular, and more maintainable specification.

Spec Changes: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1692/files