metric-backend: Define the consolidation function names once in shared typing

The consolidation function wire names were spelled out in the
graph-designer schema and were about to grow more copies. They now live
in their own shared-typing schema, one definition per function group
(gauge, sum, histogram, histogram-preserve), from which the TypeScript
and Python types are generated; the graph-designer schema references
them.

CMK-36907

Change-Id: Ia4227f5aae4e43c73dfa1f5b3c5eb6470d90b6ba