Move the nginx client creation into the TcpBorderClient and HttpBorderClient

[ciroque]

Is your feature request related to a problem? Please describe

No problem.

Synchronizer should not know about the NGINX Plus client. It should simply provide an HttpClient to the NewBorderClient factory method.

Describe the solution you'd like

In order to facilitate extensibility for various Border Server types, no assumptions should be made about which underlying client is used. The BorderClient implementation should contain that knowledge (e.g.: The HttpBorderClient and TcpBorderClient each know they need the NGINX Plus client, they can create those using the given HttpClient).

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.

[ciroque]

BorderClient::NewBorderClient should take an HttpClient instead of interface {} for the borderClient parameter.