feat: add support for backend-specific parameters

[emmanuelmathot]

Summary

Adds **kwargs support to load_collection() to allow passing additional backend-specific parameters that are not part of the standard openEO specification.

Motivation

Different openEO backends may support additional parameters like nodata, target_crs, tile_buffer, etc. This change enables users to leverage these backend-specific features.

Example

cube = connection.load_collection(
    "sentinel-2-l2a",
    spatial_extent={"west": 10, "south": 45, "east": 11, "north": 46},
    temporal_extent=["2023-01-01", "2023-06-01"],
    bands=["B04", "B03", "B02"],
    nodata=0,           # backend-specific
    target_crs=32632,   # backend-specific
)

Changes

• Added **kwargs to Connection.load_collection() and DataCube.load_collection()
• Extra parameters are passed through to the process graph
• Added tests and updated documentation