Add support for Raspberry Pi

[andrei-gavrila]

The path defined here for Linux platform references the Debian 64bit AWS Session Manager Plugin. For Raspberry Pi the URL should have _arm64 instead of _64bit as documented here: https://docs.aws.amazon.com/systems-manager/latest/userguide/install-plugin-debian.html

This is the offending line, simply replacing it will fix the issue on Raspberry Pi but it's not the proper fix :) I'm not that familiar with the platform module and I have no idea how to properly identify the architecture so I can't propose a proper fix:

https://github.com/xen0l/aws-gate/blob/52f7281af2805e28d44ee3906b14d48acb5201fb/aws_gate/constants.py#L52