feat: Enable static tls config in Webtransport

[MarcoPolo]

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We have some code to support a valid TLS cert in the webtransport listener, but we disabled it in https://github.com/libp2p/go-libp2p/pull/1927. There's some subtlety around making it secure.

In the mean time we may bake some assumptions that a webtransport multiaddr mostly has a certhash: https://github.com/libp2p/go-libp2p/pull/2227#discussion_r1151324966. When we re-enable static tls config we should revisit that discussion and see if anything needs to be changed

[marten-seemann]

Blocked on https://github.com/libp2p/specs/issues/507.

[MarcoPolo]

Thank you! I couldn't remember that issue