Allow arbitrary MySQL JDBC connection properties in config.yml

storage.mysql.properties is now a free-form map merged into the JDBC
URL as ?key=value&key=value..., letting hosts that require TLS set
things like sslMode: REQUIRED, or override the useSSL/autoReconnect
defaults, without needing a code change for every possible parameter.

README.md

@@ -104,7 +104,11 @@ cross-world travel in the original Stargate mod.
 ## Cross-server (Velocity / Bungee)
 
 1. Set `storage.type: mysql` in every backend server's `config.yml` and point them at
-   the **same** database — this is how servers see each other's gates.
+   the **same** database — this is how servers see each other's gates. Add any
+   connection parameters your host requires (TLS, timezone, etc.) under
+   `storage.mysql.properties` - each key becomes `?key=value` on the JDBC URL. It
+   defaults to `useSSL: false` / `autoReconnect: true`; list a key here to override
+   that default (e.g. `useSSL: true` plus `sslMode: REQUIRED`).
 2. Give each backend a unique `server-id` in `config.yml` that matches its name in
    the proxy config (`velocity.toml` / `config.yml` servers list).
 3. Set `cross-server.enabled: true` on every backend.