REST api: made it possible to copy the REST headers from REST requests to the corresponding TransportRequest(s)

Introduced the use of the FilterClient in all of the REST actions, which delegates all of the operations to the internal Client, but makes sure that the headers are properly copied if needed from REST requests to TransportRequest(s) when it comes to executing them.

Added new abstract handleRequest method to BaseRestHandler with additional Client argument and made private the client instance member (was protected before) to force the use of the client received as argument.

The list of headers to be copied over is by default empty but can be extended via plugins.

Closes #6513