api: Add ChromeSdk.FindLkgm buildAPI endpoint

The endpoint exposes the logic to find available ChromeOS image version
for a given board equal to or older than the LKGM.

Here is an example input for manual testing with gen_call_scripts:
{
  "build_target": {
    "name": "eve"
  },
  "chrome_src": "/home/user/work/chromium/src",
  "fallback_versions": 10
}

BUG=b:271041696
TEST=manual test using gen_call_scripts

Change-Id: I50a432638bb932821099e33a120845bf7d14b99a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4788811
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Tested-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Cindy Lin <xcl@google.com>
diff --git a/api/router.py b/api/router.py
index 32761ab..7e53f8d 100644
--- a/api/router.py
+++ b/api/router.py
@@ -25,6 +25,7 @@
 from chromite.api.gen.chromite.api import artifacts_pb2
 from chromite.api.gen.chromite.api import binhost_pb2
 from chromite.api.gen.chromite.api import build_api_pb2
+from chromite.api.gen.chromite.api import chrome_lkgm_pb2
 from chromite.api.gen.chromite.api import copybot_pb2
 from chromite.api.gen.chromite.api import depgraph_pb2
 from chromite.api.gen.chromite.api import dlc_pb2
@@ -608,6 +609,7 @@
     router.Register(api_pb2)
     router.Register(artifacts_pb2)
     router.Register(binhost_pb2)
+    router.Register(chrome_lkgm_pb2)
     router.Register(copybot_pb2)
     router.Register(depgraph_pb2)
     router.Register(dlc_pb2)