Adding custom headers capability to REST client

Adding custom headers capability to REST client

For YANG PATCH requests to ODL to work, they need to have a Content-
type header of application/yang.patch+json and should not have Accept
as application/json

Current REST client inserts a default header to the requests with this
content:

  Content-Type: application/json
  Accept: application/json

The solution was to add the possibility of sending custom headers alon-
gside the other parameters.

Change-Id: I2cf0cd2ef7b87f4f5a246d427dffafe266cb33f7
Issue-ID: CCSDK-926
Signed-off-by: ottero <rodrigo.ottero@est.tech>