Enable application to bind to a callback that will inject
headers to the requests sent to the NSX. This can for example
be: 'X-NSX-EUSER' which will provide the NSX context of the user
on behalf of whom the opertaion is done.
A new method set_inject_headers_callback(in is added. This method
receives a callback that should return a dictionary of headers to
be added.
Change-Id: I90fa7ea8c7828bf97aec66321e4169588108760e
216d8a3be9