FIP: External cast data

FIP "External cast data" by @vrypan and @davidfurlong proposes enabling long casts by storing their content at an external URI rather than on Farcaster hubs, and embedding it using Embed.url. The proposal supports only plain text long casts and is designed to be extended later to other data types such as location, audio and events without hub changes. Long-cast JSON-LD payloads can be embedded in an HTML page or hosted stand-alone under an fc: URI scheme, and the payload must be less than 32kb. The authors state that the 32kb limit allows content publishable as a 25,000-character long tweet, and hub requirements and storage requirements are unchanged.