Add dbus proxy

[mbssrc]

Description

This PR adds two modules:

1. dbus-proxy module as a wrapper around xdg-dbus-proxy, which connects either system or session bus to a local socket, with filtering and policy
2. socket proxy module that connects givc-agent instances. One of these run as server (with dbus-proxy enabled) that connects to the local socket, and the other as client where applications can connect to a socket and thus talk to the remote dbus

Other changes:

• streamline submodules so they can be re-used across the code base
• update flake & smaller fixes

Checklist

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Test procedure added to nixos/tests
• Author has run nix flake check --accept-flake-config and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status

Testing

Tests added to simulate various scenarios. Test on Ghaf and PR in works.

[slakkala]

Overall looks nice (not a Go expert though)

Some miscellany comments and questions added, nothing major.

[slakkala]

👍