wamr-wasi-extensions: add a cmake package to provide our wasi extension (#4344)

* wasi_ephemeral_nn.h: add a convenience wrapper header
* wamr-wasi-extensions: add a cmake package to provide our wasi extension

the sample app was tested with:
* wasmtime
* iwasm with https://github.com/bytecodealliance/wasm-micro-runtime/pull/4308

currently only contains wasi-nn.
maybe it makes sense to add lib-socket things as well.

cf. https://github.com/bytecodealliance/wasm-micro-runtime/issues/4288

wamr-wasi-extensions/test.sh

@@ -0,0 +1,19 @@
+#! /bin/sh
+
+# Copyright (C) 2025 Midokura Japan KK.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+PREFIX=/tmp/wamr
+WASI_SDK=${WASI_SDK:-/opt/wasi-sdk}
+
+cmake -B build-lib \
+-DCMAKE_TOOLCHAIN_FILE=${WASI_SDK}/share/cmake/wasi-sdk.cmake \
+-DCMAKE_INSTALL_PREFIX=${PREFIX} \
+.
+cmake --build build-lib -t install
+
+cmake -B build-app-nn \
+-DCMAKE_TOOLCHAIN_FILE=${WASI_SDK}/share/cmake/wasi-sdk.cmake \
+-DCMAKE_PREFIX_PATH=${PREFIX} \
+samples/nn
+cmake --build build-app-nn