support live content (LL-HLS partial segments with byte ranges, RFC 8673)

[scottlamb]

Currently use of http_serve::{serve, Entity} requires that the length be known in advance. However, there are useful scenarios in which this is not true:

• https://www.akamai.com/blog/performance/-using-ll-hls-with-byte-range-addressing-to-achieve-interoperabi
• https://datatracker.ietf.org/doc/html/rfc8673

What sort of interface extension / mechanism would http_serve need to support this?