Reuse SmartGson.getReader() in JenkinsCiPluginsRepository

JenkinsCiPluginsRepository was reading directly from a URI when fetching
a remote stream rather than relying on the SmartGson.getReader(), which
had exactly the same logic.

Use a unique channel to fetch from a remote source so that mocking
becomes easier and also replacing it with a different transport handler
would not require changes to the JenkinsCiPluginsRepository
implementation.

Change-Id: Ib1f25e7085cafb1c133e1b7e48dc993328489254
(cherry picked from commit b6c8fa3ea94abfdde067621ce71fbb3a52ba26b6)