wasi-nn: fix context lifetime issues (#4396)

* wasi-nn: fix context lifetime issues

use the module instance context api instead of trying to roll
our own with a hashmap. this fixes context lifetime problems mentioned in
https://github.com/bytecodealliance/wasm-micro-runtime/issues/4313.

namely,

* wasi-nn resources will be freed earlier now. before this change,
  they used to be kept until the runtime shutdown. (wasm_runtime_destroy)
  after this change, they will be freed together with the associated
  instances.

* wasm_module_inst_t pointer uniqueness assumption (which is wrong
  after wasm_runtime_deinstantiate) was lifted.

as a side effect, this change also makes a context shared among threads
within a cluster. note that this is a user-visible api/abi breaking change.
before this change, wasi-nn "handles" like wasi_ephemeral_nn_graph were
thread-local. after this change, they are shared among threads within
a cluster, similarly to wasi file descriptors. spec-wise, either behavior
should be ok simply because wasi officially doesn't have threads yet.
althogh i feel the latter semantics is more intuitive, if your application
depends on the thread-local behavior, this change breaks your application.

tested with wamr-wasi-extensions/samples/nn-cli, modified to
call each wasi-nn operations on different threads. (if you are
interested, you can find the modification at
https://github.com/yamt/wasm-micro-runtime/tree/yamt-nn-wip-20250619.)

cf.
https://github.com/bytecodealliance/wasm-micro-runtime/issues/4313
https://github.com/bytecodealliance/wasm-micro-runtime/issues/2430

* runtime_lib.cmake: enable WAMR_BUILD_MODULE_INST_CONTEXT for wasi-nn

as we do for wasi (WAMR_BUILD_LIBC_WASI)

build-scripts/runtime_lib.cmake

@@ -106,6 +106,7 @@ endif ()
 
 if (WAMR_BUILD_WASI_NN EQUAL 1)
     include (${IWASM_DIR}/libraries/wasi-nn/cmake/wasi_nn.cmake)
+    set (WAMR_BUILD_MODULE_INST_CONTEXT 1)
 endif ()
 
 if (WAMR_BUILD_LIB_PTHREAD EQUAL 1)