Allow passing a custom rustls ClientConfig

[praveenperera]

electrum-client currently creates its own rustls::ClientConfig for ssl:// connections using webpki_roots. The public Config API doesn’t seem to let callers provide their own ClientConfig or certificate verifier.

This is a problem for mobile apps that want to use native OS certificate validation, for example via rustls-platform-verifier on Android/iOS. In Cove, this would help fix UnknownIssuer errors for certs trusted by the platform store. Related issue in cove: bitcoinppl/cove#776

[oleonardolima]

related to #137.

[oleonardolima]

Does the issue still happens with the validate_domain option as false ? (though, it'd be only a workaround)

I agree, in order to properly fix this, we'd need to expose the rustls::ClientConfig API as mentioned #137.