Support more complex `parameters` schemas

Currently `MCPTool`'s `parameters` field has to be a `Vector{ToolParameter}` https://github.com/JuliaSMLM/ModelContextProtocol.jl/blob/12d42e9a6f83e6e47423319368261d9482aa28cb/src/features/tools.jl#L44, which is defined as https://github.com/JuliaSMLM/ModelContextProtocol.jl/blob/12d42e9a6f83e6e47423319368261d9482aa28cb/src/features/tools.jl#L14-L19. This doesn't handle more complex types such as `array`, `enum`, or `object`. I've patched the package locally to accept arbitrary schemas and all appears fine in use with multiple clients. Would you be happy @kalidke for support to be widened to accept arbitrary schemas? Otherwise tool parameters are pretty limited in practice.