webtransport: put custom WebTransport code in the (basic)host and observed address manager

[p-shahi]

          We could do the most minimal thing here, which is a bit hacky:
             1. Add a helper func `RemoveCerthashes`.
             2. ObservedAddrManager Calls `RemoveCerthashes` before comparing things.
             3. basic_host will copy cerrthashes to webtransport addrs before returning multiaddrs from `Addrs`. This way conusmers of an addr get the correct and up to date webtransport addrs.
          It's hacky because 1 & 3 involve these components having special knowledge of the whole system. But it would probably be the fastest and smallest short term fix. And we could write a test for the behavior to protect against a future regression.

Originally posted by @MarcoPolo in #2233 (comment)