Client: Add websocket client and more control over the cert selection

[roosterfish]

This PR adds a new client Websocket function that allows upgrading a HTTP connection using either the local or remote client.

Furthermore another RemoteClientWithCert app function is added which allows providing a custom remote server cert in case the remote is not using the common cluster cert. This is helpful during the pre-init phase (MicroCloud explicit trust establishment) as well as when trying to connect to additional servers making use of mTLS.

[roosterfish]

@masnax do we also want another Websocket function in the client/client.go file?