Draft: buffered datagram API sketch

[dtikhonov]

Summary

• add buffered datagram API types and callbacks
• add engine settings for buffered datagram max size and default drop policy
• split per-connection setters/getters for buffer size vs drop policy, plus queueing APIs
• add QUIC DATAGRAM max payload size getter (HTTP max already exists as lsquic_stream_get_max_http_dg_size())

Testing

• not run (API/header-only change)