Trim matrix secret when read from file

Treat the secret the same as synapse when read from file a file, so that
the same secret file can be reused between mas and synapse.

https://github.com/element-hq/synapse/blob/v1.137.0/synapse/config/experimental.py#L50

crates/config/src/sections/matrix.rs

@@ -131,7 +131,11 @@ impl MatrixConfig {
     /// Returns an error when the shared secret could not be read from file.
     pub async fn secret(&self) -> anyhow::Result<String> {
         Ok(match &self.secret {
-            Secret::File(path) => tokio::fs::read_to_string(path).await?,
+            Secret::File(path) => {
+                let raw = tokio::fs::read_to_string(path).await?;
+                // Trim the secret when read from file to match Synapse's behaviour
+                raw.trim().to_string()
+            }
             Secret::Value(secret) => secret.clone(),
         })
     }