linux-sgx: Implement SGX IPFS as POSIX backend for file interaction (#1489)

This PR integrates an Intel SGX feature called Intel Protection File System Library (IPFS)
into the runtime to create, operate and delete files inside the enclave, while guaranteeing
the confidentiality and integrity of the data persisted. IPFS can be referred to here:
https://www.intel.com/content/www/us/en/developer/articles/technical/overview-of-intel-protected-file-system-library-using-software-guard-extensions.html

Introduce a cmake variable `WAMR_BUILD_SGX_IPFS`, when enabled, the files interaction
API of WASI will leverage IPFS, instead of the regular POSIX OCALLs. The implementation
has been written with light changes to sgx platform layer, so all the security aspects
WAMR relies on are conserved.

In addition to this integration, the following changes have been made:
 - The CI workflow has been adapted to test the compilation of the runtime and sample
    with the flag `WAMR_BUILD_SGX_IPFS` set to true
 - Introduction of a new sample that demonstrates the interaction of the files (called `file`),
 - Documentation of this new feature

README.md

@@ -151,6 +151,7 @@ The WAMR [samples](./samples) integrate the iwasm VM core, application manager a
 
 - [**basic**](./samples/basic): Demonstrating how to use runtime exposed API's to call WASM functions, how to register native functions and call them, and how to call WASM function from native function.
 - **[simple](./samples/simple/README.md)**: The runtime is integrated with most of the WAMR APP libraries, and a few WASM applications are provided for testing the WAMR APP API set. It uses **built-in libc** and executes apps in **interpreter** mode by default.
+- **[file](./samples/file/README.md)**: Demonstrating the supported file interaction API of WASI. This sample can also demonstrate the SGX IPFS (Intel Protected File System), enabling an enclave to seal and unseal data at rest.
 - **[littlevgl](./samples/littlevgl/README.md)**: Demonstrating the graphic user interface application usage on WAMR. The whole [LVGL](https://github.com/lvgl/lvgl) 2D user graphic library and the UI application are built into WASM application.  It uses **WASI libc** and executes apps in **AOT mode** by default.
 - **[gui](./samples/gui/README.md)**: Move the [LVGL](https://github.com/lvgl/lvgl) library into the runtime and define a WASM application interface by wrapping the littlevgl API. It uses **WASI libc** and executes apps in **interpreter** mode by default.
 - **[multi-thread](./samples/multi-thread/)**: Demonstrating how to run wasm application which creates multiple threads to execute wasm functions concurrently, and uses mutex/cond by calling pthread related API's.