rbw: add systemd service for rbw-agent and add sshAgent option to make rbw the ssh agent

[minionflo]

Description

add systemd service for rbw-agent and add sshAgent option to make rbw the ssh agent

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -A dev --run treefmt.

• Code tested through nix build .#test-all
  or a targeted nix run .#tests -- <pattern>.

• Test cases updated/added. See example.

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  • Basic tests added. See Tests

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[ambroisie]

TIL rbw can be used as an SSH agent.

[bmrips]

I recommend adding test cases. To test the sshAuthSock configuration, take inspiration from the tests for other SSH agents like e.g. ssh-agent.

[minionflo]

I recommend adding test cases. To test the sshAuthSock configuration, take inspiration from the tests for other SSH agents like e.g. ssh-agent.

I sadly do not know how to write tests and do not have the energy or time for learning how to write tests