refactor(openeo): split gateway into modules, extend /processes, refresh docs

[PondiB]

Summary

• Refactor: Replace monolithic earthgrid-core/src/openeo.rs with earthgrid-core/src/openeo/:
  • mod.rs — crate boundary + re-exports
  • types, graph, output, execute, catalogue, api, tests
  • geoprocess — temporal period labels, reducers, resample_spatial via gdalwarp CLI
• API surface: Remove earthgrid_core::openeo_geoprocess; use earthgrid_core::openeo::geoprocess instead.
• GET /processes: Add aggregate_temporal_period and resample_spatial entries so discovery matches openeo::execute_sync and openEO Python/R list_processes().
• Docs: README — openEO Python/R examples (data window, fetch_metadata=False, R con for processes / compute_result). STATUS — current layout, tests, known issues (minimal collection metadata, gdalwarp on PATH). AGENTS — openeo/ in project tree; testing note 148 passing + 1 ignored.

Motivation

• Easier maintenance than a single very large openeo.rs.
• Client libraries and /processes should advertise the same process ids the backend runs.

Testing

• cargo build --release
• cargo test — 148 passed, 1 ignored (gdalwarp roundtrip; optional: cargo test gdal_warp_same_crs -- --ignored when gdalwarp is on PATH).

[PondiB]

@MatMatt - I refactored openEO into earthgrid-core/src/openeo/ submodules + geoprocess for warp/temporal bits, aligned /processes with execution, and refreshed docs.