Register "libp2p" ALPN with IANA

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In order to safely use the "libp2p" ALPN with TLS connections, we should register it with IANA.

Registration requests should be sent to the mailing list described
in [RFC 8447, Section 17]. If approved, designated experts should
notify IANA within three weeks. For assistance, please contact
iana@iana.org.

https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids