metric-backend: Add consolidation function to the custom-query config

The custom-query config gains the consolidation function a query runs,
the agent-side Query, the form's stored-value models, and the vue schema
carry the field. The ruleset migration stamps configs predating the
field with `gauge_last`.

The agent-side Query also picks up the three histogram fraction
thresholds the form already stores, so the histogram functions have
their parameters at hand once the agent runs them.

CMK-36906

Change-Id: Ia7e720ddb78f523bd984a55f0f09f81173a20688