Add macaroon_secret_key_path config option (#17983)

Another config option on my quest to a `*_path` variant for every
secret. This time it’s `macaroon_secret_key_path`.

Reading secrets from files has the security advantage of separating the secrets from the config. It also simplifies secrets management in Kubernetes. Also useful to NixOS users.

docs/usage/configuration/config_documentation.md

@@ -3091,6 +3091,22 @@ Example configuration:
 ```yaml
 macaroon_secret_key: <PRIVATE STRING>
 ```
+---
+### `macaroon_secret_key_path`
+
+An alternative to [`macaroon_secret_key`](#macaroon_secret_key):
+allows the secret key to be specified in an external file.
+
+The file should be a plain text file, containing only the secret key.
+Synapse reads the secret key from the given file once at startup.
+
+Example configuration:
+```yaml
+macaroon_secret_key_path: /path/to/secrets/file
+```
+
+_Added in Synapse 1.121.0._
+
 ---
 ### `form_secret`