feature: Customizable client-side connection id and non-20-byte connection id support for shorter connection id for smaller overhead

This issue is technically consists of two features, but targeting single problem so writing as single issue.

I'm planned to use quiche with unreliable extension for real-time communication and want to reduce packet size overhead.
Therefore I want to use shorter connection id for client side (and reject longer in on_conn_established).

However, currently tokio-quiche doesn't support changing length of connection id in `InboundPacketRouter` and doesn't allow specifying scid or cid generator.

Therefore I want to add support changing length of connection id to tokio-quiche.

Start by reading tokio-quiche's InboundPacketRouter and the on_conn_established path to understand where connection ID lengths and generators are selected. Define how client-side IDs can use a configurable shorter length, how custom SCIDs or CID generators are supplied, and how longer IDs are rejected; done means both behaviors work for the unreliable-extension use case.

Written by the indexing model from the issue text.