add a set of apis to configure wasi-nn via InstantiationArgs2

[zhanheng1]

add a set of apis to configure wasi-nn via InstantiationArgs2:

1. Add a new command line option for wasi-nn: --wasi-nn-graph=encoding:target:model_path1:model_path2:...:model_pathN;
2. Add new structs to manage this cmd option: WASINNGlobalContext and wasi_nn_graph_registry;
3. Add 2 new parameters for load_by_name(encoding and target). load_by_name() can get encoding and target from WASINNGlobalContext and set into backend.
4. Update test codes.