Fix metadata leak in private room message forwarding

[dozro]

Description

fix metadata leak when forwarding from a private room by setting 'm.mentions' and 'm.relates_to' explicitly to 'null'

Fixes #190

Example

{
  "content": {
    "body": "(Forwarded message from a private room)\n\n> oi3fhif3h2qoifew",
    "format": "org.matrix.custom.html",
    "formatted_body": "<div data-forward-marker><p>(Forwarded message from a private room)</p><blockquote>oi3fhif3h2qoifew</blockquote></div>",
    "m.mentions": null,
    "m.relates_to": null,
    "moe.sable.message.forward": {
      "is_forwarded": true,
      "original_event_private": true,
      "original_timestamp": 1773267714441,
      "v": 1
    },
    "msgtype": "m.text"
  },
  "event_id": "$pM0XU5end_0rfHK4e54jZq0dfZHdivUgUxCiDZalTNI",
  "origin_server_ts": 1773267722160,
  "sender": "@client-testing:itsrye.dev",
  "type": "m.room.message",
  "unsigned": {
    "age": 23622,
    "transaction_id": "m1773267721992.3"
  },
  "room_id": "!ySMjK3zfhIHXNVlNDS:itsrye.dev"
}

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings