Support `multipart/byteranges` for 206 responses

[handrews]

RFC 9110 §14.6 defines the multipart/byteranges media type for use in 206 (Partial Content) responses. We should see what, if anything, is needed to support it. Probably something involving the Encoding Object.

Some clients apparently used a not-entirely-compatible multipart/x-byteranges, but I'd argue that RFC 9110 carries enough weight that we should support multiparte/byteranges even if the old x- type is currently more broadly used.