Integrate WASI-NN into WAMR (#1521)

Initial integration of WASI-NN based on #1225:
- Implement the library core/iwasm/libraries/wasi-nn
- Support TensorFlow, CPU, F32 at the first stage
- Add cmake variable `-DWAMR_BUILD_WASI_NN`
- Add test case based on Docker image and update document

Refer to #1573

build-scripts/config_common.cmake

@@ -291,3 +291,6 @@ if (WAMR_BUILD_SGX_IPFS EQUAL 1)
     add_definitions (-DWASM_ENABLE_SGX_IPFS=1)
     message ("     SGX IPFS enabled")
 endif ()
+if (WAMR_BUILD_WASI_NN EQUAL 1)
+  message ("     WASI-NN enabled")
+endif ()