Workaround invalid AT protocol URIs from BridgyFed

BridgyFed generates AT protocol URIs like at://did:plc:... that violate
RFC 3986 URL syntax due to colons in the authority part. This causes
parsing failures in Link.fromJsonLd() and other vocabulary methods.

Added temporary fix to URL-encode colons in did:plc DIDs to make them
valid URLs (at://did%3Aplc%3A...) until upstream issue is resolved.

Fixes https://github.com/fedify-dev/fedify/issues/436