Add support for file sharing

Currently the project can exchange consent between the sender and the receiver. The next step is to start the file transfer once the receiver approves the file. You can checkout the [sendFile](https://github.com/spectre10/fs-cli/blob/main/session/send/handlers.go#L111) function in [fs-cli](https://github.com/spectre10/fs-cli) repository for reference. Also handle the case where the receiver refuses to receive the files.