Add a feature in FreeCC to pass a file from container to the host

[ouankou]

To be safe, the file should be passed to /tmp and never be executed.
Then the file can be downloaded from frontend by user.

[ouankou]

PWD has a handler to get a file from the container. We don't need to implement a new function.
https://github.com/freeCompilerCamp/play-with-compiler/blob/a8e345233629088d3a618c531a5436a2dabfb3b3/handlers/bootstrap.go#L82

The definition is located in: https://github.com/freeCompilerCamp/play-with-compiler/blob/master/handlers/file_instance.go

To use this function, assume we open a tutorial and create test.txt in the home folder /home/freecc. For any sandbox, a session id and a instance id will be generated automatically. The following GET request will get /home/freecc/test.txt

curl --request GET lab.freecompilercamp.org:5010/sessions/bsk4ctgq4uig0s44264g/instances/bsk4ctgq_bsk4ctoq4uig0s442650/file?path=%2Fhome%2Ffreecc%2Ftest.txt

The URL pattern is <pwc_url>/sessions/<session_id>/instances/<instance_id>/file?path=<encoded file path>.
The parameter at the end is to indicate the demanding file. The file path after url encoding is the value of the key path.
The returned file content is encoded to Base64 format.