step-agent module

[Srylax]

This is still very WIP.

Add a new module to start the agent as a systemd service

Questions:

• Would DynamicUser be an option for the systemd service
• Does /etc/step-agent/agent.yaml need to be writeable (if not we could create a config for NixOS)
• Should the NUR template be preserved (e.g. move back packages to the toplevel and nixosModules->modules)

TODO:

• Update the systemd config to the latest upstream
• passthrough the latest packages as default
• add swtpm service
• fix mainProgram = "step-agent";
• test with npins
• test with flakes
• test on virtualized / software tpm
• update the add packages line (if we decide to move to .packages)
• update README on the usage

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