expose negotiated TLS 1.3 namedgroup in TlsContext / SecurityParameters

[jschauma]

It can be desirable for the TLS server to know which NamedGroup was negotiated for a given connection. I don't believe there currently exists a way to extract this information from a TlsContext.

For this, it might be useful to add a SecurityParameters.getNegotiatedGroup() similar to SecuriyParameters.getNegotiatedVersion() or SecurityParameters.getCipherSuite().