TLSNextProto specifies how the Transport switches to an alternate protocol (such as HTTP/2) after a TLS NPN/ALPN protocol negotiation. If Transport dials an TLS connection with a non-empty protocol name and TLSNextProto contains a map entry for that key (such as "h2"), then the func is called with the request's authority (such as "" or "") and the TLS connection. The function must return a RoundTripper that then handles the request. If TLSNextProto is nil, HTTP/2 support is enabled automatically.