FDB-528: FamCatalogue and FamStore backends via OpenFAM / OpenFAM Mock

[mcakircali]

Description

Implements FamCatalogue for FAM catalogue stack (reader/writer wiring, index persistence, engine discovery), and tests. It uses eckit::FamList and eckit::FamMap.

Persists and loads DB key from FAM map storage.
Index listing by iterating catalogue map entries.
per-DB catalogue map and global FAM DB registry.
FamIndex: lazy creation/selection and archive writes.
Flush behavior sorts index axes in batch.
Axis computation by merging axes from loaded indexes.

FamIndex: lookup, iteration, and axis reconstruction from persisted fam.
Entries in FamMap with payload:

• timestamp
• polymorphic FieldLocation
• datum Key

FamEngine:
Read root FAM URI from config fam_roots.
Implements canHandle and visitableLocations via FAM registry.
Discover catalogue locations from both Key and MarsRequest queries.

Notes:
Catalogue & index map names are truncated because of OpenFAM object-name limits.
The catalogue map uses underscore-prefixed reserved keys for internal metadata filtering.
Catalogue registration is done by a global FAM registry map for engine discovery.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/fdb/pull-requests/PR-250