Make it possible to use the REF for monitoring model runs

## The motivation

There is an interest in using the REF for monitoring model runs https://github.com/ESMValGroup/Community/discussions/215#discussioncomment-12969238 and it seems quite suitable for that.

## The proposed solution

In order to make this work also with data that has not been CMORized yet, it should be easy to add custom data sources. It should be possible to do this without modifying the climate-ref, climate-ref-core, and other infrastructure packages. We would also need a how-to describing how to do this.

An HPC executor #142 would be very useful for this task too.

## Alternatives

Everyone writes their own scripts to:
1) see what data is available
2) generate ESMValTool recipes based on that
3) run those recipes on HPC

This is not very attractive because it duplicates effort.

## Additional context

Start by reading this issue, the linked Community discussion, and issue #142 about an HPC executor. The payload names no files, tests, or entry points, so the first task is to define the custom data-source boundary and how monitoring runs should use it without modifying the infrastructure packages. Done should include a documented how-to and an agreed implementation scope.

Written by the indexing model from the issue text.