TransportClient: Mark transport client as such when instantiating

This allows plugins to load/inject specific classes, when the client started
is a transport client (compared to being a node client).

Closes #7552
Introduced a transient context to the rest request

Similar to the one in `TransportMessage`. Added the `ContextHolder` base class where both `TransportMessage` and `RestRequest` derive from

Now next to the known headers, the context is always copied over from the rest request to the transport request (when the injected client is used)