[FEATURE]: add an interface for file storage

[Narayanbhat166]

The code files that are retrieved from the leetcode servers are to be stored in a system. Since we are just storing and retrieving the files as it, this need not be stored in a file system. Also the access patterns and the way in which these files are stored as a flat structure instead of a hierarchical one defeat the purpose of storing these files in a file system. The usual procedure is as follows

• Retrieve the code data of a submission
• Hash the code to find duplicate entries ( yes, there are people who just submit the same code )
• Store the hash code in a database if the hash does not already exist.

The file storage interface should have two methods

async fn store_file(&self, file_contents: String, file_name: String) -> Result<(), FileStorageInterfaceError>;
async fn get_file(&self, file_name: String) -> Result<String, FileStorageInterfaceError>;

This interface can be implemented for many backends. For example the files can be stored in a discord server, or on a telegram server. There can be a local database which maintains mapping of where a particular file is being stored.

The files can also be batched and then stored in order to make the storage efficient.