Adding DiscoFeedEndpoint feature

This feature switches to a different Feed Endpoint when enabled.

* changed FeedNetwork send functions to send signed-out requests
whenever the gaia passed-in is empty. This obviates the force sign-out
request parameter, and allows SendApiRequest to send signed out requests

Bug: b/183932700
Change-Id: I2e442c4687875305f788adbc0351faecb61cf6df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2818523
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#872032}