Add support for IntentionalMentions to EditedContent.mediaCaption.

[pixlwave]

When building the content for an edited reply in Element X, for a regular m.text message we're able to call with_mentions to bundle any intentional mentions into the final event. However this isn't available when using the new create_caption_edit method and it's returned EditedContent.mediaCaption value. Ideally the SDK would support this as we're using the same composer for both types of edit.