Add endpoint and cron job to aggregate all dimensions and values

appengine/swarming/swarming_rpcs.py

Index: appengine/swarming/swarming_rpcs.py
diff --git a/appengine/swarming/swarming_rpcs.py b/appengine/swarming/swarming_rpcs.py
index 9f3c667e6e1863676273d594f02abe9f07d722ed..1d3754ed37e9884769b8ba443b6fefc2c98dc5e9 100644
--- a/appengine/swarming/swarming_rpcs.py
+++ b/appengine/swarming/swarming_rpcs.py
@@ -383,6 +383,11 @@ class BotsCount(messages.Message):
   busy = messages.IntegerField(5)
 
 
+class BotsDimensions(messages.Message):
+  """Returns all the dimensions and dimension possibilities in the fleet."""
+  bots_dimensions = messages.MessageField(StringListPair, 1, repeated=True)
+
+
 class BotEvent(messages.Message):
   # Timestamp of this event.
   ts = message_types.DateTimeField(1)