Add HTTPS file server Demo

The demo shows how to run the unmodified HTTPS file server with Occlum.

demo/https_server/.gitignore

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+mongoose_src/
+deps/
+occlum_workspace/
+simplest_web_server_ssl
+server.*

demo/https_server/README.md

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+# Use Mongoose HTTPS file server with Occlum
+
+This project demonstrates how to run a HTTPS file server with [Mongoose Embedded Web Server Library](https://github.com/cesanta/mongoose).
+
+Step 1: Download and build Mongoose and OpenSSL, then build the sample HTTPS file server shipped with Mongoose's source code
+```
+./download_and_build_mongoose.sh
+```
+When completed, the resulting file server can be found at `./mongoose_src/examples/simplest_web_server_ssl/simplest_web_server_ssl`.
+
+Step 2: You can run the HTTPS file server either on Occlum
+```
+./run_https_server_in_occlum.sh
+```
+or on Linux
+```
+./run_https_server_in_linux.sh
+```
+The HTTPS file server should now start to listen on port 8443 and serve HTTPS requests.
+
+Step 3: To check whether the HTTPS server works, run
+```
+curl -k https://127.0.0.1:8443
+```
+in another terminal.
+
+It is also possible to access the HTTPS server directly in a Web browser. But if you are testing in a Docker container, you won't be able to open the URL `https://127.0.0.1:8443` in a browser on the host OS. To fix this, you have to manually map port 8843 of the Docker container to a port on the host OS. Check out how to use [the `-p` argument of `docker run` command](https://docs.docker.com/engine/reference/commandline/run/).

demo/https_server/download_and_build_mongoose.sh

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+#!/bin/bash
+set -e
+
+# 1. Download and install OpenSSL 1.1.1
+mkdir -p deps/openssl
+pushd deps/openssl
+git clone https://github.com/openssl/openssl .
+git checkout tags/OpenSSL_1_1_1 -b OpenSSL_1_1_1
+CC=occlum-gcc ./config \
+  --prefix=/usr/local/occlum/x86_64-linux-musl \
+  --openssldir=/usr/local/occlum/ssl \
+  --with-rand-seed=rdcpu \
+  no-zlib no-async no-tests \
+  -fPIC -pie
+make -j
+sudo make install
+popd
+
+# 2. Download Mongoose 6.15
+mkdir -p mongoose_src
+pushd mongoose_src
+git clone https://github.com/cesanta/mongoose .
+git checkout tags/6.15 -b 6.15
+popd
+
+# 3. Build the https server example in mongoose
+pushd mongoose_src/examples/simplest_web_server_ssl
+CC=occlum-gcc CFLAGS_EXTRA="-Wno-format-truncation -fPIC -pie" make
+popd

demo/https_server/run_https_server_in_linux.sh

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+#!/bin/bash
+export LD_LIBRARY_PATH=/usr/local/occlum/x86_64-linux-musl/lib:$LD_LIBRARY_PATH
+https_server=simplest_web_server_ssl
+set -e
+
+# 1. Copy files
+cp -f mongoose_src/examples/simplest_web_server_ssl/$https_server .
+cp -rf mongoose_src/examples/simplest_web_server_ssl/server.* .
+
+# 2. Run https_server
+./$https_server

demo/https_server/run_https_server_in_occlum.sh

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+#!/bin/bash
+https_server=simplest_web_server_ssl
+set -e
+
+# 1. Init Occlum Workspace
+rm -rf occlum_workspace
+mkdir occlum_workspace
+cd occlum_workspace
+occlum init
+
+# 2. Copy files into Occlum Workspace and Build
+cp ../mongoose_src/examples/simplest_web_server_ssl/$https_server image/bin
+cp -r ../mongoose_src/examples/simplest_web_server_ssl/server.* image
+cp /usr/local/occlum/x86_64-linux-musl/lib/libssl.so.1.1 image/lib
+cp /usr/local/occlum/x86_64-linux-musl/lib/libcrypto.so.1.1 image/lib
+occlum build
+
+# 3. Run https_server
+occlum run /bin/$https_server