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:

aws-gate/aws_gate/constants.py

Line 52 in 52f7281

"download": SSM_PLUGIN_BASE_URL + "/ubuntu_64bit/session-manager-plugin.deb",