Add parsing tokens from files or environment variables

[ShiromMakkad]

Info

This adds support for token parsing from environment variables with the format RATHOLE_{server name in uppercase}_TOKEN or RATHOLE_DEFAULT_TOKEN. It also adds token_file and default_token_file to client and server configs. The updated Readme has a more detailed description of the changes.

Testing

Tested using cargo test. I've also added a file inside valid_configs to test for the file token and a new test case for environment variable parsing.

Todo

• Update the Docker Hub Readme with environment variable parsing.
• Update the Chinese version of the Readme.