secrets(role): introduce secrets role for storing secrets

Allows storage of secrets to then be referenced in other places.
The motivation was storing WireGuard secrets for systemd-networkd.

roles/secrets/README.md

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+# Role `secrets`
+
+Allows storing the given secret contents in the configured files.
+
+## Supported Distributions
+
+Should work on Debian-based distributions.
+
+## Required Arguments
+
+None.
+
+## Optional Arguments
+
+- `secrets__secrets`: List of secrets.
+  Defaults to the empty list (`[ ]`).
+- `secrets__secrets.*.name`: (File)name for the secret (in the `/etc/ansible_secrets` directory).
+- `secrets__secrets.*.content`: The secret content to store.
+- `secrets__secrets.*.owner`: The owner of the secret file.
+  Defaults to `root`.
+- `secrets__secrets.*.group`: The group of the secret file.
+  Defaults to `root`.
+- `secrets__secrets.*.mode`: The mode of the secret file.
+  Defaults to `0640`.