Generate social sharing thumbnail

[sondreb]

Sharing links on Blockchair will display a more informative preview of the transaction or block, we should add similar functionality to greatly improve the social sharing experience on social media.

Example:

Depending if the social sharing specification allows external URL query or not (same as origin or not), this logic must either be implemented on Indexer or Explorer. It should probably be on Indexer, and if same as origin is required (link in metadata), then the explorer can simply forward request.