Initial version of payload build_api endpoint

This is the initial version of the payload build_api endpoint,
and should be considered a work in progress as we fill out the
functionality/tests and get the existing production infrastructure
to a state which will be compatible. The end state is to have
this be called by a recipe that originates in a swarming
task created by a builder.

Further down the road this allows us to decouple the payload
step entirely from the legacy builders.

BUG=chromium:1000894
TEST=./tests

Change-Id: Iee7b27fd416aa24d04f77475cb60e444bcfe41b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1910360
Tested-by: George Engelbrecht <engeg@google.com>
Auto-Submit: George Engelbrecht <engeg@google.com>
Reviewed-by: Alex Klein <saklein@chromium.org>
diff --git a/api/router.py b/api/router.py
index 32f7b98..8908fe5 100644
--- a/api/router.py
+++ b/api/router.py
@@ -27,6 +27,7 @@
 from chromite.api.gen.chromite.api import depgraph_pb2
 from chromite.api.gen.chromite.api import image_pb2
 from chromite.api.gen.chromite.api import packages_pb2
+from chromite.api.gen.chromite.api import payload_pb2
 from chromite.api.gen.chromite.api import sdk_pb2
 from chromite.api.gen.chromite.api import sysroot_pb2
 from chromite.api.gen.chromite.api import test_pb2
@@ -326,6 +327,7 @@
   router.Register(depgraph_pb2)
   router.Register(image_pb2)
   router.Register(packages_pb2)
+  router.Register(payload_pb2)
   router.Register(sdk_pb2)
   router.Register(sysroot_pb2)
   router.Register(test_pb2)