Allow to customise http.Transport for RPC client

[fyfyrchik]

Allow to customise http.Transport for RPC client

[roman-khimov]

Providing client TLS certificate.

It was meant to be supported by the client natively (see Options), but never was implemented. Same thing with server authentication. But you know that and other cases like routing really can require something more fancy. We'll think about it.

[fyfyrchik]

Turns out func(h *http.Client) is not that flexible. For Websocket clients we need to explicitly provide some options in NewWS because it connects differently

neo-go/pkg/rpcclient/wsclient.go

Line 456 in b66cea5

dialer := websocket.Dialer{HandshakeTimeout: opts.DialTimeout}

Explicit *tls.Config option solves mTLS issues, but again other points in the OP require custom options.