This is needed so clients can modify tool schemas, and not merely to
wrap ToolHandlers.
(A consequence of returning a copy of Tool instead of modifying it.)
Replace all occurrences of ServerRequest[*CallToolParams] and other
concrete instantiations with CallToolRequest and the like.
Make the XXXRequest types aliases, to preserve the convenience of
generics for the internal machinery (see shared.go, for example.)
I will expand the aliases in a followup PR.
