Expand LP2PQuicTransport interface

[backkem]

This PR expands the LP2PQuicTransport support.

• I added a receiver.ontransport to handle in coming transports.
• I decided to make the LP2PConnection optional to establish a LP2PQuicTransport. Instead this is now done from the Request/Receiver. Naturally, all permission/authentication/grant steps remain relevant. However, this reduces boilerplate code when you're only interested in establishing a LP2PQuicTransport.
• I removed the start() method of the LP2PQuicTransport to be more inline with the WebTransport interface design.

Follow-up discussion based on these changes:

• DataChannel vs WebTransport #29
• API Design: EventHandler vs ReadableStream #30

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[anssiko]

This is a good design direction.

Btw. Happy Holidays! 🎄