Add support for alt-text for videos

[MrPetovan]

At the moment the alternative text of videos attached to Mastodon isn't included in the Friendica post display.

Source public Mastodon post: https://toot.chez.gay/@rob/112398313888256091
Source object:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "ostatus": "http://ostatus.org#",
      "atomUri": "ostatus:atomUri",
      "inReplyToAtomUri": "ostatus:inReplyToAtomUri",
      "conversation": "ostatus:conversation",
      "sensitive": "as:sensitive",
      "toot": "http://joinmastodon.org/ns#",
      "votersCount": "toot:votersCount",
      "blurhash": "toot:blurhash",
      "focalPoint": {
        "@container": "@list",
        "@id": "toot:focalPoint"
      }
    }
  ],
  "id": "https://toot.chez.gay/users/rob/statuses/112398313888256091",
  "type": "Note",
  "summary": null,
  "inReplyTo": null,
  "published": "2024-05-07T06:09:22Z",
  "url": "https://toot.chez.gay/@rob/112398313888256091",
  "attributedTo": "https://toot.chez.gay/users/rob",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "cc": [
    "https://toot.chez.gay/users/rob/followers"
  ],
  "sensitive": false,
  "atomUri": "https://toot.chez.gay/users/rob/statuses/112398313888256091",
  "inReplyToAtomUri": null,
  "conversation": "tag:toot.chez.gay,2024-05-07:objectId=689645:objectType=Conversation",
  "content": "<p>Eeeeek!</p>",
  "contentMap": {
    "en": "<p>Eeeeek!</p>"
  },
  "attachment": [
    {
      "type": "Document",
      "mediaType": "video/mp4",
      "url": "https://toot.chez.gay/system/media_attachments/files/112/398/312/679/254/559/original/5bc779b0c0e82ccb.mp4",
      "name": "Video shows Rob in his kitchen. In ISL he signs: “Good morning. Today is my first day in my nee job. Excited/nervous! See you later.  Bye” then he blows a kiss to the camera. ",
      "blurhash": "UKH-lbs:%Lxu?[kBjrof9bxuD*RQoPR+RkWC",
      "width": 1080,
      "height": 1920
    }
  ],
  "tag": [],
  "replies": {
    "id": "https://toot.chez.gay/users/rob/statuses/112398313888256091/replies",
    "type": "Collection",
    "first": {
      "type": "CollectionPage",
      "next": "https://toot.chez.gay/users/rob/statuses/112398313888256091/replies?only_other_accounts=true&page=true",
      "partOf": "https://toot.chez.gay/users/rob/statuses/112398313888256091/replies",
      "items": []
    }
  }
}

Mastodon HTML

<video src="https://toot.chez.gay/system/media_attachments/files/112/398/312/679/254/559/original/5bc779b0c0e82ccb.mp4" poster="https://toot.chez.gay/system/media_attachments/files/112/398/312/679/254/559/small/5bc779b0c0e82ccb.png" preload="none" role="button" tabindex="0" aria-label="Video shows Rob in his kitchen. In ISL he signs: “Good morning. Today is my first day in my nee job. Excited/nervous! See you later.  Bye” then he blows a kiss to the camera. " title="Video shows Rob in his kitchen. In ISL he signs: “Good morning. Today is my first day in my nee job. Excited/nervous! See you later.  Bye” then he blows a kiss to the camera. " lang="en" volume="1" style="width: 100%;"></video>