Customizable variadic length connection id for client

[anatawa12]

Closes #2541

[zaczh]

Verified this against my use case — a MASQUE proxy on tokio-quiche 0.19.1 with an 8-byte server CID generator. With scid_len = 8 CONNECT-UDP round-trips fine including near-MTU datagrams; without it short-header packets get misrouted at the hardcoded MAX_CONN_ID_LEN, so the handshake completes but no data flows.

One API thought: scid_len duplicates info the router can already reach. It holds the SharedConnectionIdGenerator, so the length could be derived from the generator instead of a separate field the caller has to keep in sync:

// defaulted method on ConnectionIdGenerator
fn cid_len(&self) -> usize { MAX_CONN_ID_LEN }

then short_dcid(buf, self.cid_generator.cid_len()). Drops the mismatch footgun the field doc already warns about, and stays non-breaking. Needs to be a method rather than an assoc const since the generator is used behind dyn.

[anatawa12]

@zaczh Thank you for feedback! My first use case was reducing client scid so directly using cid_len from ConnectionIdGenerator was not possible, but we have ConnectionId itself so we changed the code to derive scid_size from actual ConnectionId for client and ConnectionIdGenerator for server endpoint instead providing as separated parameter