node: Support systemd credential for passphrase

While it is possible to pass the passphrase via the environment, e.g.
`EnvironmentFile=<path to file that contains "RAD_PASSPHRASE=…">`
this is less secure than passing it via a file, because the environment
is inherited down the process tree.

Thus, allow using a systemd credential. The ID of the credential must be

    xyz.radicle.node.passphrase

and is not user-configurable.

Passing the passphrase via file is now possible with
`LoadCredential=xyz.radicle.node.passphrase:<path to file that contains passphrase>`

This requires just a bit of plumbing in `radicle-node`.

Because this mechanism is more secure than using the environment
variable `RAD_PASSPHRASE`, it takes priority. That is, if both the
systemd credential is available, *and* the environment variable
`RAD_PASSPHRASE` is set, the former is preferred.

Heads-up:
 1. The contents of the file must be valid UTF-8 (see documentation of
    `std::fs::read_to_string`). Assuming that the passphrase is at some
    point chosen by the user and typed on a keyboard, this does not
    seem like a severe restriction.
 2. The contents of the file are not processed otherwise, i.e. line
    breaks (notably at the end of the file) are not stripped.

The related `issue/8bd040e9de05e7fc27e373ebc1649ff4ad930e7a` asked for a
very similar feature: Passing the passphrase via a file named by the
value of the of the environment variable `RAD_PASSPHRASE_FILE`.
It was also briefly discussed at
<https://radicle.zulipchat.com/#narrow/channel/369277-heartwood/topic/.60RAD_PASSPHRASE_FILE.60/with/529104447>.

CHANGELOG.md

@@ -16,11 +16,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `rad issue` now uses `clap` to parse its command-line arguments.
    This affects error reporting as well as help output.
 - `radicle-node` now supports systemd Credentials (refer to
-  <https://systemd.io/CREDENTIALS> for more information) to load
+  <https://systemd.io/CREDENTIALS> for more information) to load:
-  the secret key, in addition to the commandline argument
+    1. The secret key, in addition to the commandline argument
        `--secret` (higher priority than the credential) and the
        configuration file (lower priority than the credential).
        The identifier of the credential is "xyz.radicle.node.secret".
+    2. The optional passphrase for the secret key, in addition to the
+       environment variable `RAD_PASSPHRASE` (lower priority than the
+       credential).
+       The identifier of the credential is "xyz.radicle.node.passphrase".
 
 ## Fixed Bugs
 

crates/radicle-node/src/main.rs

@@ -235,7 +235,28 @@ fn execute(options: Options) -> Result<(), ExecutionError> {
     log::info!(target: "node", "Version {} ({})", env!("RADICLE_VERSION"), env!("GIT_HEAD"));
     log::info!(target: "node", "Unlocking node keystore..");
 
-    let passphrase = profile::env::passphrase();
+    let passphrase = None;
+
+    #[cfg(all(feature = "systemd", target_os = "linux"))]
+    let passphrase = passphrase.or_else(|| {
+        const ID: &str = "xyz.radicle.node.passphrase";
+        match radicle_systemd::credential::path(ID) {
+            Err(err) => {
+                log::warn!(target: "node", "Failed to obtain path of the passphrase file via systemd credential with '{ID}': {err}");
+                None
+            },
+            Ok(Some(ref path)) => match std::fs::read_to_string(path) {
+                Ok(passphrase) => Some(passphrase.into()),
+                Err(err) => {
+                    log::warn!(target: "node", "Failed to read passphrase from '{}': {err}", path.display());
+                    None
+                }
+            }
+            Ok(None) => None,
+        }
+    });
+
+    let passphrase = passphrase.or_else(profile::env::passphrase);
 
     let secret_path = options.secret;