GeoZarr Overviews Extension

[christophenoel]

This Overviews extension adapts the work proposed by @emmanuelmathot in EOPF Explorer – Data Model PR #41 to align with the GeoZarr specification.

The following key adaptations have been introduced:

• The structure and metadata are defined in an abstract manner, using the groups, variables, and attributes components of the Common Data Model (CDM) in the dedicated section (clause 7).
• The decision to apply multiscaling at the group level is retained but clarified through the definition of a variable group and its typical contents within the CDM.
• The Zarr encoding (derived from the general CDM-to-Zarr mapping) is explicitly defined in Clause 9 for implementation clarity.
• The former group and from_group fields were replaced by the more neutral ìdand optionalpath` field, which preserves referential meaning without binding to a specific data model structure.
• An optional tile_matrix_ref field was added to support datasets aligned with external map tiling definitions.

NOTE: As the referenced PR is substantial, its approval should be understood as agreement with the general concept it introduces. The detailed aspects will be refined and, where necessary, discussed further in dedicated follow-up pull requests.