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/nn/CMakeLists.txt

@@ -0,0 +1,25 @@
+# Copyright (C) 2025 Midokura Japan KK.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+add_library(wamr-wasi-nn INTERFACE)
+
+set(wasi_nn_header_dir ${CMAKE_CURRENT_SOURCE_DIR}/../../core/iwasm/libraries/wasi-nn/include)
+
+set(headers
+    ${wasi_nn_header_dir}/wasi_ephemeral_nn.h
+    ${wasi_nn_header_dir}/wasi_nn.h
+    ${wasi_nn_header_dir}/wasi_nn_types.h
+)
+
+set_property(TARGET wamr-wasi-nn PROPERTY PUBLIC_HEADER ${headers})
+
+target_include_directories(wamr-wasi-nn
+                           INTERFACE
+                           $<BUILD_INTERFACE:${wasi_nn_header_dir}>
+                           $<INSTALL_INTERFACE:include>)
+
+install(TARGETS wamr-wasi-nn
+        EXPORT wamr-wasi-nn-config
+        PUBLIC_HEADER DESTINATION include/wamr)
+install(EXPORT wamr-wasi-nn-config
+        DESTINATION lib/cmake/wamr-wasi-nn)