Enable PDS change app view CDN url

[lovemyliwu]

Current, in the real word, the China Fire Wall had blocked bluesky cdn domain, I can only read plain text from my PDS.

I saw PDS_BSKY_APP_VIEW_CDN_URL_PATTERN can config, but only effect to local view, as atproto design PDS is the only endpoint for client, I think we really need to control the cdn url when app view rendering.

I also create a issue to tracking this problem: #3018